Where to enter Product Key

CCleaner for Mac
1

I was receiving an error that I could not use the product at my location, so I removed CCleaner and reinstalled it (installed version is now 1.17.603), on a Ma.

There is not an option to enter my current Product Key (valid until 04/14/2020), only Upgrade.

Is there a workaround or do I need to purchase the product again?

2

It's simple when you know how:

Go to Options>About and click on 'Upgrade to Pro.'

Enter your key and name in the box that comes up and click on 'Register' to re-register your existing key.