Module is not compatible with Drupal 8

The core issue is that version 8.x-1.4 of the File Entity Browser module is using a library (once) that was introduced in Drupal 9 and is not available in Drupal 8.9.x. To resolve the issue, you can either downgrade to version 1.3 of the module, manually add the once library. Upgrading to Drupal 9 is another long-term solution that will avoid these kinds of problems in the future.

I am avare of all that, but module should drop support for Drupal 8 if it doesn't work with it.