I would suspect MathJax not being properly installed. However some informations are missing to provide a correct answer. Could you please give us some informations so that someone can try to reproduce your problem:
- which notebook did you use (Sage notebook or jupyter notebook) ?
- did you use the command line ?
- which version of Sage did you use ?
- which OS ?
- did you install Sage from the binaries, and which ones ?
- did you compile Sage yourself ?
- which command did you type precisely to get the error ?
- which error message did you get ?
- ... ?