121: Missing whitespace between $$ and math label
A math block starts and ends with two dollar signs $$
. A math block can be accompanied by a label after the closing $$
and whitespace. In your case, a whitespace character is missing.
Solution
Make sure to put whitespace between the closing $$
and a math label. A single space character as in the following example is enough.
$$x=\frac{1}{x}+1$$ #(1)