I am trying to use the Build Accented Glyph feature as was suggested in #283 (comment).

In this font, we already have the lowercase ‘a’ (u+0061) and the standard 'acute' (u+00b4) character but not the ‘acutecomb’ glyph (u+0301).

Using FontForge-2020-03-14-67687b0-x86_64.AppImage on Ubuntu 18.04.4 LTS, going to the ‘a acute’ character slot (u+00e1) and using the ‘Element > Build > Build Accented Glyph’ function as per the instructions does not work because the ‘Element > Build' menu is greyed out.

http://designwithfontforge.com/en-US/Diacritics_and_Accents.html says:

So, using the example of building the ‘a acute’ character (u+00e1) , we would need to have already created the lowercase ‘a’ (u+0061) and the ‘acutecomb’ glyph (u+0301). Then selecting the ‘a acute’ character slot and using the ‘Element > Build > Build Accented Glyph’ function, FontForge will place a reference to the lowercase ‘a’ glyph and a reference to the ‘acutecomb’ glyph into the ‘a acute’ character slot (see below).

It should be noted here that if you are not using anchors and lookup tables to position diacritic marks, then, if the glyph of a particular diacritic mark is not present in your font, FontForge will instead use a similar spacing character in place. For example, if the combining mark 'acutecomb' (u+0301) is not present, then FontForge will use the standard 'acute' (u+00b4) character when it automatically builds any acute accented glyphs. If the 'acutecomb' is present, then FontForge will always use that, unless you specifically force FontForge to use spacing characters for building accented glyphs.