We now have a district-wide license for Canva and this occurs if you had already setup an account. Please take the following steps to resolve the issue.
First Clear Browsing Data and reset settings
Then log in with their school accounts and passwords, NOT using the 'continue with Google'. Then you click on left hand side, you should see both accounts.
Here is a link to more troubleshooting tips - please open a help desk ticket if you are still having trouble.