Welcome to MusicBrainz. Thank you for adding this new Artist and Release to MusicBrainz.
You might want to take a break for 12 hours and come back. When you add a new Artist to MusicBrainz, it takes some time before this Artist is reflected in the indexes. So, until then, when you type the new Artist name in the Release Artist field of the Add Release page, MusicBrainz can't find the new Artist. That means you are trying to add a Release without a Release Artist, and MusicBrainz prevents that.
You don't have to wait. There is an option in the search results menu to do a "direct search". The direct search bypasses the indexes, and can find an Artist which you just added. However, if you don't see the direct search option quickly, it might be easier simply to wait for the indexes to discover the new Artist entry.
By the way, when asking for help, it is good to put in links to what you have done. That way we can look and offer advice.
Is Anita Samsom the Artist you are working on? Or, Artist/2410d, to use the MBID, abbreviated?