I can no longer find the product on its developer's website, so I don't have an official Windows compatibility list for QuickBooks Customer Manager v2.5. However, according to various other sources, the latest Windows edition supported by the application is Windows Vista.
If you want to run it on Windows 10, you might still be able to do that either by launching QuickBooks Customer Manager in compatibility mode with Windows XP/Vista or by running the application through a virtual machine.
If it were me, I would look for an alternative solution such Pipedrive, Insightly or even Inuit's own QuickBooks Online.