I get this error pop-up when saving down from G3 to G2. I’ve already changed the Font Info > Other > File Saving > File format version > Glyphs 2. And get this error (see image).
OS Sonoma 14.0
Glyphs 3.1.2 (3151)
Any help would be welcome 
Go into your font info, open the Masters tab. Check the metrics for the masters and deactivate any scopes you might have set for a metric.
why do you need to save as Glyphs 2?
A colleague is using that version.
I have removed the Old Style and Small Cap scopes from the Font Info > Masters > Metrics. All seems to now be working.
The “small cap” scope should work.
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