Oops. Re-read your first post. You have a dual band set up. Yes the FPU will not allow you to enter test mode. I guess it just can't handle two radios at once. So now you have to jump through some hoops.
As an example, I'll assume VHF is radio one and UHF is radio two. Read the radio and save the archive.
Disconnect the UHF radio two control cable. Open a blank archive in FPU, change model to single head and single band VHF. Create a dummy channel. Insure there are some correct VHF test frequencies in its menu. Write this to the radio (ignore "model mismatch" message). You can now enter test mode and make adjustments to that radio.
Now it gets worse. You'll need to swap the KRK-7DB ends and put the one from radio one onto radio two.
Follow the above except program as a UHF single band single head. You can now tune the UHF.
Reassemble as original, program with your saved archive (again ignore "model mismatch") and you are good. A real PITA. Goes a little easier if you have it all out on a bench, but then you'll need a spare set of cables.
I have never done front panel tuning on a dual band X90. I don't know if you'll have to do the same kind of thing or what.