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)