There could be many reasons why this is occurring, including a slow internet connection or server maintenance. However, typically, the problem is caused by using an incompatible browser or related to caching issues.
Printify currently works best in Google Chrome, and our Mockup Generator is not designed for mobile functionality.
Clear the cache and cookies of your current browser (here’s how to do it for Google Chrome).
Try using a different internet browser — preferably Google Chrome, if not using it already.
Try logging in from a different device.