Latest news 2021-09-06: new blog post "Legacy Documents and TeX Live Docker Images".

Only a certain type of character may be used as a delimiter (for example, ( ) [ ]), check which one you have specified. This error may also occur if you have forgotten a \right or not used it in the same scope. (Remember to use a . if you want an invisible delimiter) or you may have forgotten to end your array environment with \end{array}.