Hello @ChristinaRatlef 

The videos could have issues on the app depending on how your teacher embedded them and what type of format they are and/or what type of external tool they used. Most videos should play on the app but occasionally the video file type or external tool used for them can have issues. 

If you're using the Canvas App, we recommend clearing the cache: 

https://community.canvaslms.com/t5/Canvas-Student-iOS-Guide/What-Student-app-options-can-I-manage-fr... 

If none of that appears to fix it, it would be best if you contact Canvas Support and they can take a look at the specifics of your course and assignment and provide better insight.

https://community.canvaslms.com/t5/Canvas-Basics-Guide/How-do-I-contact-Canvas-Support/ta-p/389767