Cashback offers usually have a time/date before which you can’t apply.
Everybody has to wait untill that date before they can apply
The button(s) on the application will not be active before that date.
Check the Terms and Conditions on the cashback offer which should give that date as the start of the ‘Claim Period’.
Once that date has passed then there is also a FULL 24 hours after purchase that you should wait
See: How it works
That 24 hrs is needed for your purchase to get fully registered at the company running the cashback promotion.
Until your purchase is fully registered the casback firm won’t recognise it.
If you try to apply earlier than the offer terms state then it won’t let you.
Check your purchase confirmation email, there should be a ‘How to claim your cashback’ link in it which will tell you how long you need to wait before applying..
PS. That ‘Form is not available in your language’ error for the CCleaner support form is known about. Once you have the error it won’t even let you go back to English.
Close the webpage, open the English form again, and use the English form without trying to change language.