Hi. I have the same problem, so I'd like to know : what exactly did you format ? microSD card (for vita2sd) ? Sony memory card for vita ? Did you reset the console ??? Did you loose activation ? Thank you.
In my case, I have a 64GB Sony memory card (bought from Japan) almost full, and I bought a 128GB Samsung Evo Plus microSD card to install as ux0.
Maybe my problem was that I first tried with a 8GB microSD that I had lying around, and so I first tried to install this one as uma0. It didn't succeed, but I didn't use a complete guide that time I guess (no img used before formatting the 8gb microSD in windows as exfat).
All of it. I formatted that can be formatted and it worked.