I can’t export any font that has mkmk feature with glyphs 3 the exporting issues appre like the attached screen, but If I disabled the mkmk it works well, Also, the exporting issues are not valid or right, can you fix this
Did you write the mkmk feature code in File → Font Info… → Features? Generally, you should not need to write that code manually since Glyphs generates the mkmk feature code behind the scenes at export.
yes, I write some exceptional situations in File → Font Info… → Features manually to avoid any overlapping between two letters marks, as you see in the attached image I added an end code line to make sure all good
What version of Glyphs do you have?
3.0.3 
Can you update to the latest cutting edge version (activate it in Preferences > Updates).
@GeorgSeifert still the same error, when I try to export with mkmk feature, but when I deactivate it, it is export successfully
@GeorgSeifert @FlorianPircher I realized that the issue happened when I use both of mark and mkmk features, but when I use only one of them, it is export well