\nsim is listed in table 69 of the Comprehensive LaTeX Symbol List, under the caption
AMS Negated Binary Relations
In section 1.1 of the document, we find
“AMS” means to use the AMS packages, viz.
Actually most of the symbols with that tag require
amssymb and this is the case for
Note that the result of
\not\sim is rather different, so the final answer is
... bla bla bla $a \nsim b$ bla bls ...