If you get a
Something went wrong warning while you try loading a font file into Fontself, this may have 2 different causes:
The font file was not generated with Fontself Maker. It's just not possible to edit OpenType fonts that were exported or converted a different tool than Fontself Maker (like as a TTF TrueType font file). Editing raw font files usually leads to technical glitches when not done appropriately, and in most cases the font licenses may not allow to modify them. So it's for both technical and legal reasons. 😉
The font file is legit but Illustrator needs a complete reboot. Sometimes Illustrator's extension technology scrambles and only a fresh relaunch of the operating system can clean its memory. As quitting and reopening the app itself may not suffice, restarting the computer is the best solution.