incompatible with factorio 0.18.31-0.18.35. versions before 0.18.35 may also has this question
Mods to disable:mods load fail：boblibrary/technology-functions.lua:455: bad argument #1 of 4 to 'gsub' (string expected, got table)
stack traceback:
[C]: in function 'gsub'
boblibrary/technology-functions.lua:455: in function 'get_redundant_prerequisites_smart_cached'
boblibrary/technology-functions.lua:476: in function 'prerequisite_cleanup'
boblibrary/data-final-fixes.lua:1: in main chunk

As mentioned in the discord, this isn't an issue with base factorio, it's likely another mod you have installed causing it to crash.
However, the function in question is crashing because it doesn't validate that the prerequisite entry is a string before passing it to gsub, so I've released an update that does that.
Also, difficulty split for technology functions, but unrelated to this bug.