If saving early and often doesn't help. There could be an underlying issue.
The most common reason for someone not being able to save their project is when a a new module has been added to the company's module library and the power tolerance value was input incorrectly.
If you created the new module, then you should be able to update easily. If you did not, then you will need to work with whomever added it to make updates to it.
Below are the steps for determining if this is the issue.
1. Go into your module library and go to the module you're using.
2. Change the power tolerance from "x" to 0/"x".
3. If it allows you to save it to the existing module, great! If it prompts you to save it as a different module, add something to the name, and then save.
4. Go into your model and delete any viewsheds that are placed.
5. Go into the array tab and make sure the correct panel is selected (the same one if you didn't need to add a new module, or the new module you created).
6. You should now be able to save at this point
7. Start replacing viewsheds. You should still be able to save when complete.