Greyed or disabled checkout button is most of the time caused by a conflict with another app.
Disable the app test
First, try to disable DiscountDuck to be sure the issue is linked to the app. If after disabling the app the issue is still here, it can't be created by the app, DiscountDuck doesn't execute anything on your store when disabled.
Disable incompatible apps
make sure you don't have an incompatible app installed and if you find one, try to disable it to see if it fix the problem
Contact us
If after following these steps you still have the issue, please contact us, we'll investigate the problem with you.
