Thank you for contacting the Canvas Community. We're sorry to hear you've been experiencing issues with using quizzes and assignments. If the add option in the top right corner isn't there you may want to check course settings. Are the participation dates still valid? What term is the course set to? Sometimes, the schools disable teachers from creating more content in a course after a term ends (presuming you're a teacher and not a TA, etc.) in which case you'll probably want to contact your school directly.
If it isn't a setting, It is likely your internet browser and we recommend clearing the cache and cookies or trying another browser.
On your computer, open Chrome.
At the top right, click the three dots
On the left, choose Privacy and Security
Then, clear browsing data
At the top, choose a time range. To delete everything, select All time. I suggest at least 4 weeks
Next to "Cookies and other site data" and "Cached images and files," check the boxes.
Click Clear data.