I get the above warning in Overleaf.

their 'Get Error Assist' is not helpful as it tells me to add 'amssymb' package but it is already there.

@CroCo are you trying to use typewriter in maths mode?

@cfr what do you mean by typewriter?

@CroCo Font shape OMS/cmtt/m/n' undefined` suggests you're trying to use a font from family cmtt in maths because OMS is a maths encoding. cmtt is a 'typewriter' font, usually monowidth. the kind of thing \ttfamily or \texttt produces in text. that isn't usually used with maths encodings.

@mickep fyi: 1 bug is fixed in microtype; 1 bug was already fixed in gs. (don't know when gs updates will actually circulate, but ...)

@cfr Very good!

@mickep there is something very ... odd ... about how microtype implements letter spacing for luatex with unicode fonts. there's another question now, where it again produces weird results, but I can't figure out why :(. is it very different doing letter spacing with luatex versus xetex, do you know? if it were just a unicode/type1 difference ... I guess luaotfload is the complication, but I don't really get it.

the previous bug would work well with @DavidCarlisle's most noteworthy package.

Spotted update of easybook!

@mickep it is not a luatex issue. it is definitely microtype.

@cfr Ok, "good"

@egreg I hope it is the one you've been waiting for.

@cfr Sorry, I don't read Chinese… ;-)

@egreg 你可以问语言专家

@DavidCarlisle Like Google translate?

@cfr I've looked at their updated version, it seems I need to add \usepackage[T1]{fontenc} which solved the problem.

@egreg Cyfieithu Gŵgl?

@cfr :-D