Cannot delete my staging site

When I try to delete my staging site I get this message:

Something went wrong! Error: Permission Error: Can not delete file /home/xxxxx/public_html/staging/wp-content/deleteme.c20dbb32350d483296d1127e684867be.php. Please try the WP Staging Small Server Settings or submit an error report.

I have tried the recommended settings – doesn’t work.
I have also tried to delete these files (I have 4 “deleteme” files) – but they come back as soon I have deleted them.

What should I do to delete my staging site?

Thanks.