oauth-private.key does not exist or is not readable

So, I imported another project from Bitbucket and tried to launch it using php artisan serve, I always get this error:

[LogicException]                                                                   
  Key path "file:///var/www/html/DesignViewer5/storage/oauth-private.key" does not   
  exist or is not readable                                                           

I don't get this error when I make a project myself, I can't run any other command. I tried 'php artisan key:generate', and got the exact same error.

I tried: composer update, and got this:

Loading composer repositories with package information
Updating dependencies (including require-dev)
Package operations: 0 installs, 1 update, 0 removals
  - Updating spatie/laravel-permission (1.11.1 => 1.12.0) Downloading: 100%         
Writing lock file
Generating autoload files
> Illuminate\Foundation\ComposerScripts::postUpdate
> php artisan optimize


  [LogicException]                                                             
  Key path "file:///var/www/html/DesignViewer5/storage/oauth-private.key" doe  
  s not exist or is not readable                                               


Script php artisan optimize handling the post-update-cmd event returned with error code 1

Anyone knows how to fix it? Thanks!

via O'Niel