An Error in Math mode

What is the meaning of

\mathcal allowed only in math mode


What sign should I use to fix this error?**

• Welcome to TeX.SE. Here there is the answer: tex.stackexchange.com/questions/15662/what-is-math-mode – Sebastiano Jan 12 at 21:47
• It denotes the math calligraphic font, and as its name suggests, it is reserved for mathematics. There exists calligraphic fonts that can be used in text mode (rsfs or brushscript, for instance), and you can use them if you load the relevant package. – Bernard Jan 12 at 21:50
• You probably have something like blah blah \mathcal{A} blah blah in your text. Instead you should have blah blah $\mathcal{A}$ blah blah. – Werner Jan 12 at 23:36