Try downloading version 13 from here:
Then it will update itself when required inside the app, and allow you to install version 15 on Mac OS 10.12.6 (this worked on my 10,12.6 system, mid 2011 iMac)
It's very sad that TeamViewer has restricted their software to run only on 10.13.6 onward because many older Mac systems cannot update any further than 10.12.6.
I tried to install TW with teh dmg presente in official website in Juin 8 2020.
My system is OS 10.12.6.
And in the installation I couldn t finish the process, please see my print screen.