Maybe you want to check this:
[Problem with ocm-provider Verzeichnis /var/www/html/nextcloud/ocm-provider/ existiert nicht mehr in NC 27.1.2]
My conclusion from that is, that this is a false warning message, the checking tool does seem to be outdated. So I decided for me to ignore it for now and wait for an update by the Nextcloud team.
I have the same problem in NC 28.0.1 and had it until some minutes ago in NC 27.1.2.
According to the link it should disappear with NC 27.1.3 which I never installed however.