not found in include_once()

Any resolution for this issue? I'm also facing the same issue.

Not sure this applies to everyone getting the error, but I was getting this error after enabling css preprocessing. The error went away when css preprocessing was disabled, but I didn't want to leave it that way. Turns out that I somehow had the bootstrap theme, but it wasn't being managed by composer.

Running composer require drupal/bootstrap solved the issue for me and allows css preprocessing.

#5 does the job for me too.

see comment #5