Hello EddieRobertson1, welcome to the Microsoft community, I'll be happy to help you today;
I understand that you cannot save files in Paint3D;
Initially I recommend that you try the following;
Check for new updates on Windows Update:
1: Open the Windows 10 "Start" menu and select the "Settings" item.
2: then select the "Update and Security" option.
3: in the "Windows Update" section, click "Check for updates".
4: If there are no updates, the message "You are up to date" will be displayed.
5: if there is any pending system update, the message "Features update for Windows 10 ..." will be displayed, then click on the "Download and install" item to start the system update.
6: wait for the update download to finish and restart the PC, or schedule a restart to be able to finish the procedure.
Update Microsoft Store apps;
Open the store and click on Library;
Click on Get updates and wait for all apps to be updated;
Test and check if you can save files in Paint3D;
If you still can't try resetting the app;
Press Windows key + I and go to Applications;
Select Paint3D from the list and click advanced options;
Scroll down and click Reset;
Try saving the files in Paint3D;
Hope this helps, see you soon!