Looks like EPiServer.CMS 11.18.0 is no longer available in the nuget feed https://nuget.episerver.com/package/?id=EPiServer.CMS
You shouldn't remove published packages from the feed.
The removal of the nuget package has been announced:https://world.episerver.com/blogs/Magnus-Rahl/Dates/2020/8/episerver-cms-11-18-0-delisted-from-nuget-feed/
Looks like EPiServer.CMS 11.18.0 is no longer available in the nuget feed https://nuget.episerver.com/package/?id=EPiServer.CMS
You shouldn't remove published packages from the feed.