Give it a few hours. I pushed out a package about an hour ago that should've generated a new release file. bintray has issues like this once in a while that seem to resolve themselves.
Again this error
Err:10 https://dl.bintray.com/openmediavault-plugin-developers/usul buster InRelease
Could not resolve 'dl.bintray.com'
(...)
W: Failed to fetch https://dl.bintray.com/openmed…ul/dists/buster/InRelease Could not resolve 'dl.bintray.com'