Undefined Control Sequence with imakeidx

the problem here is the presence of a | in a string that is being written into an index entry.

the | has a special function in index entries: it substitutes for (and is interpreted as) a backslash for commands that are relevant to the indexing process, such as see (input as |see in index{...}) or the "brackets" that indicate a page span, |( and |).

the proper way to input this character in an index entry depends on whether it is in math or text.

in text, a | can be "quoted" by preceding it with a ditto mark: "|.

in math, the proper substitution is vert, or, if the string to appear in the index output is ||, then Vert. since these are often paired, lvert ... rvert or abs{...} (or the equivalents for the double forms) are often more appropriate.

Barbara Beeton pointed out (https://tex.stackexchange.com/a/326635/220244) that | has a special function in index entries, which causes the problem.

This problem occurs, for example, if

index(maintopic!subtopic) 

was accidentally misspelled to

index(maintopic|subtopic)