Hi Sherry thanks for posting in the Lucidchart Community! Could you try creating the assignment in a private or incognito window to see if that works? Could you also try connecting to a different network? Sometimes account performance issues can be related to internet security/speed or browser issues. These steps will help us eliminate that possibility.
If you are still unable to create an assignment after these steps could you send a screenshot of the associated Javascript Console output after reproducing the issue via this form. You can open your Javascript Console by pressing Ctrl+Shift+J (if your computer is a Windows/Linux) OR Cmd+Opt+J (if your computer is a Mac). This will enable us to look into this in more detail in a one on one troubleshooting ticket. Thanks!
For anyone else interested in how to use our Canvas integration please take a look at the Canvas Integration Help Center article.