Which application and version are you using?
I have downloaded from URL "https://bitnami.com/stack/canvaslms/virtual-machine"
Have you installed any plugins or modified any configuration files?
What are the exact steps to reproduce the issue?
Login into canvas LMS->Click on Admin->Site-Admin->Themes->Choose any theme and click on 'open in Theme Editor".
Now scroll down to Global Navigation -> Try to change Nav Logo "select image" -> Click on Preview your changes.
getting the message
An error occurred trying to generate this theme, please try again.