Are you using the File > Plug-in Extras > Process with Topaz Photo AI or the Photo > Edit in > Topaz Photo AI workflow?
Please send me the log files so I can see what’s going on in the application. To do that, open Topaz Photo AI and go to the Help > Open Log Folder menu option and upload all the logs files in the folder.
Please also try the steps below in the meantime.
Delete any/all versions of Topaz Photo AI from your Applications folder.
Delete the plist files for Topaz Photo AI using instructions here and reboot your computer afterwards.