Hi Tom, I hope the attribute that you are trying to make searchable is an identity attribute (and not an account attribute from any source)?
From your screenshot, it looks like you are trying to make an AD account attribute as searchable which is not possible since it’s not promoted to an identity attribute at first place. I might be wrong in my assumption here!
Also the limit of 6 searchable identity attribute is IDN design decision to keep their platform on high performance, but I am not sure if this limit can be increased by working with SailPoint Support.
May be this is something which you can bring up with your CSM!