Can you try the following, this should give full permission to the reg key that the installer is attempting to update. Other users may find that it shows a different extension, the steps will be the same just select the correct extension from the list.
1, Press the Windows key + R to open the Run window
2, Type regedit and press Enter
3, Browse to HKEY_LOCAL_MACHINE\Software\Classes\
4, Locate .tga and right click and select Permissions
5, Select the Administrators group, you should be a member of this group. In the Permissions section below, opposite Full Control check the Allow box
6, Click Apply and close regedit
Now run the setup for Affinity Photo again, hopefully this time it completes. If not follow the steps again and revert the changes made.
