When I’m trying to export a variable Font I get this. Anyway the master should be compatible, as I don’t receive any red marks. Interpolation of the instances preview also works fine.
I’m having the same problem with 2 stylistic alternate letters, but it’s probably the same mistake …
It may be incompatible but not marked as such when there are no instances defined between incompatible masters. In that case add an enforce compatibility check parameter to Font Info > Font.
Or it may be the PS to TT conversion: could be a path segment that was inflecting in one master but not in another, etc. The Show Angled Handles plug-in may help spot those situations.
Also, make sure you have the latest cutting-edge version: Go to Glyphs > Preferences > Updates, activate both checkboxes and press the Update button.