Place custom Block, return Internal Server Error
When developing and encounter 500 errors, you should see if Drupal has logged the error or your webserver has logged the error. Depending on your configuration you might be able to see a full stacktrace explaining the error.

In this case you are missing a ; after your namespace declaration. So

namespace Drupal\pagebuilder\Plugin\Block

Should be

namespace Drupal\pagebuilder\Plugin\Block;

This should fix your error.