In order to download, your lab must be able to open a print dialogue box which will allow you to save the lab as a PDF. If your device is unable to print, then the button can appear inactive. This is most often the case with school controlled devices such as chromebooks and iPADs which were never set up for printing. Here are some steps you can try:
- Switch devices. If you have another device available, even a phone, this usually resolves the issue.
- Set up your device for printing. As long as you have access to your device controls, you should be able to configure it to allow for printing. This is different per device.
The other thing that can cause a lab not to be able to print is if you have an image that will not load. Here is a help article explaining what causes this and what the fix for it might be: https://support.carolina.com/what-if-my-images-are-not-uploading-in-gateway/
If the photo is preventing the lab from printing, then you will likely need to ask your professor to reopen the assignment.