Before trying some of the suggestions below, you may want to consider / share if this is a new installation or it things were working and you updated something and it broke? You gave very little context in your post.
This looks like it may be an issue that has come up recently given here, here and here. The suggestion had been to roll back to an earlier version of mistune, specifically 0.8.4, as here summarizes.
If that doesn’t help, your error traceback and here make it clear nbconvert relies on mistune, and so if you haven’t updated that recently, that may be in order. However, since that underlies a lot of the Jupyter system you may want to consider reproducing a copy of the whole environment you are working in and try updating everything in the copy and see if that fixes things. (Contemplating the best path at this point is why I added the preface of considering the larger picture here yourself, and if the next course of actions isn’t obvious, then provide some more context here so that folks can advise you. You don’t want to make things worse by redoing your environment entirely if it was some simple change you did recently that you can roll back using Anaconda, if that is what you are using to manage your Jupyter installation.)