Ok long term player here but I am struggling and yes I did do a search but cannot find the answer.

When I go to book on UA.com - I always select "refundable fares" and Premium Econ. No issue there at all. Works fine. Usually "w" fares pop up.
I can click on the "details" of the flight and see all kinds of other fare buckets and salivate.

Yes, I have "expert mode" checked

BUT if I type in ANY type of class (Y, B, M or just Y or jut M - doesn't matter) - ALWAYS get and error message - 100% of time.
Basically "fare class not available" - but again (unchecking) the specific fares (Y or B for example) when I go to the next page I can click details of the flights and see that there are say 9M or 9Y available.

What am I doing wrong?
(and yes I have expert mode checked - not that it should matter during a search (?))