I solved it thanks to this topic:
Failed to create site config directory - Troubleshooting - Checkmk Community
chown site_user:site_user /omd/sites/mysite/local/
This was the key
I solved it thanks to this topic:
Failed to create site config directory - Troubleshooting - Checkmk Community
chown site_user:site_user /omd/sites/mysite/local/
This was the key