compose.json에서 lalavel-debugar 버젼을 3.0 이상으로 해주면 됩니다.

https://github.com/barryvdh/laravel-debugbar/issues/724

[opencode@opencode www]$ composer update
Loading composer repositories with package information
Updating dependencies (including require-dev)
Your requirements could not be resolved to an installable set of packages.

  Problem 1
    - Conclusion: remove barryvdh/laravel-debugbar v2.4.3
    - Conclusion: don't install barryvdh/laravel-debugbar v2.4.3
    - Conclusion: don't install barryvdh/laravel-debugbar v2.4.2
    - Conclusion: don't install barryvdh/laravel-debugbar v2.4.1
...

composer.json 변경후...

[opencode@opencode www]$ composer update
Loading composer repositories with package information
Updating dependencies (including require-dev)
Package operations: 3 installs, 54 updates, 1 removal
  - Removing mtdowling/cron-expression (v1.2.1)
  - Updating symfony/css-selector (v3.3.6 => v4.1.0): Downloading (100%)         
  - Updating tijsverkoyen/css-to-inline-styles (2.2.0 => 2.2.1): Downloading (100%)         
  - Updating symfony/polyfill-mbstring (v1.6.0 => v1.8.0): Downloading (100%)         
  - Updating paragonie/random_compat (v2.0.11 => v2.0.12): Downloading (100%)         
  - Updating symfony/http-foundation (v3.3.6 => v4.1.0): Downloading (100%)         
  - Updating symfony/event-dispatcher (v3.3.6 => v4.1.0): Downloading (100%)         
  - Updating symfony/debug (v3.3.6 => v4.1.0): Downloading (100%)         
  - Installing symfony/polyfill-ctype (v1.8.0): Downloading (100%)         
  - Updating egulias/email-validator (2.1.3 => 2.1.4): Downloading (100%)         
  - Updating ramsey/uuid (3.7.1 => 3.7.3): Downloading (100%)         
  - Updating psr/simple-cache (1.0.0 => 1.0.1): Downloading (100%)         
  - Updating symfony/translation (v3.3.6 => v4.1.0): Downloading (100%)         
  - Updating nesbot/carbon (1.22.1 => 1.25.0): Downloading (100%)         
  - Updating league/flysystem (1.0.41 => 1.0.45): Downloading (100%)         
  - Updating erusev/parsedown (1.6.4 => 1.7.1): Downloading (100%)         
  - Updating doctrine/inflector (v1.2.0 => v1.3.0): Downloading (100%)         
  - Installing symfony/polyfill-php72 (v1.8.0): Downloading (100%)         
  - Updating symfony/var-dumper (v3.3.6 => v4.1.0): Downloading (100%)         
  - Updating symfony/routing (v3.3.6 => v4.1.0): Downloading (100%)         
  - Updating symfony/process (v3.3.6 => v4.1.0): Downloading (100%)         
  - Updating symfony/http-kernel (v3.3.6 => v4.1.0): Downloading (100%)         
  - Updating symfony/finder (v3.3.6 => v4.1.0): Downloading (100%)         
  - Updating symfony/console (v3.3.6 => v4.1.0): Downloading (100%)         
  - Installing dragonmantank/cron-expression (v2.1.0): Downloading (100%)         
  - Updating laravel/framework (v5.5.24 => v5.6.23): Downloading (100%)         
  - Updating fideloper/proxy (3.3.4 => 4.0.0): Downloading (100%)         
  - Updating maximebf/debugbar (1.13.1 => v1.15.0): Downloading (100%)         
  - Updating barryvdh/laravel-debugbar (v2.4.3 => v3.1.5): Downloading (100%)         
  - Updating doctrine/collections (v1.4.0 => v1.5.0): Downloading (100%)         
  - Updating doctrine/cache (v1.6.2 => v1.7.1): Downloading (100%)         
  - Updating doctrine/annotations (v1.4.0 => v1.6.0): Downloading (100%)         
  - Updating doctrine/common (v2.7.3 => v2.8.1): Downloading (100%)         
  - Updating doctrine/dbal (v2.5.13 => v2.7.1): Downloading (100%)         
  - Updating components/jquery (3.2.1 => 3.3.1): Downloading (100%)         
  - Updating kriswallsmith/assetic (v1.4.0 => v1.0.5): Downloading (100%)         
  - Updating eternicode/bootstrap-datepicker (v1.7.1 => v1.8.0): Downloading (100%)         
  - Updating guzzlehttp/guzzle (6.3.0 => 6.3.3): Downloading (100%)         
  - Updating intervention/image (2.4.1 => 2.4.2): Downloading (100%)         
  - Updating itsjavi/bootstrap-colorpicker (2.5.1 => 2.5.2): Downloading (100%)         
  - Updating nikic/php-parser (v3.1.2 => v4.0.1): Downloading (100%)         
  - Updating psy/psysh (v0.8.16 => v0.9.4): Downloading (100%)         
  - Updating laravel/tinker (v1.0.2 => v1.0.7): Downloading (100%)         
  - Updating ezyang/htmlpurifier (v4.9.3 => v4.10.0): Downloading (100%)         
  - Updating mews/purifier (2.0.9 => 2.1.0): Downloading (100%)         
  - Updating phpmailer/phpmailer (v6.0.2 => v6.0.5): Downloading (100%)         
  - Updating laravel/socialite (v3.0.9 => v3.0.12): Downloading (100%)         
  - Updating socialiteproviders/manager (v3.3.0 => v3.3.1): Downloading (100%)         
  - Updating socialiteproviders/kakao (v3.0.0 => v3.0.1): Downloading (100%)         
  - Updating sebastian/comparator (2.1.0 => 2.1.3): Downloading (100%)         
  - Updating doctrine/instantiator (1.0.5 => 1.1.0): Downloading (100%)         
  - Updating phpunit/phpunit-mock-objects (5.0.5 => 5.0.7): Downloading (100%)         
  - Updating phpunit/php-code-coverage (5.3.0 => 5.3.2): Downloading (100%)         
  - Updating webmozart/assert (1.2.0 => 1.3.0): Downloading (100%)         
  - Updating phpdocumentor/reflection-docblock (4.2.0 => 4.3.0): Downloading (100%)         
  - Updating phpspec/prophecy (1.7.3 => 1.7.6): Downloading (100%)         
  - Updating myclabs/deep-copy (1.7.0 => 1.8.0): Downloading (100%)         
  - Updating phpunit/phpunit (6.5.4 => 6.5.8): Downloading (100%)         
Writing lock file
Generating autoload files
> Illuminate\Foundation\ComposerScripts::postAutoloadDump
> @php artisan package:discover