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I just booked a LifeMiles ticket and had the same issue.
Make sure every box is filled in.
Usually, there is a box that says something like credit card pin code.
The US credit cards don’t have a PIN, so you can just enter anything in the box and it should work.
I put 0000 and it worked and the Complete Reservation box turned green so I could click on it.
The LifeMiles website is notoriously bad.