I had the same problem about a month ago and eventually gave up trying to purchase gems. I had entered my CC number both with and without spaces to no avail. Every entry was correct.
Today I had the same problem and saw this post. I “went back” to try changing the quantity and noticed that the CC number was in red, indicating an error in the number. I had entered the CC number including the spaces between each four numbers. I removed the spaces and it let me proceed. The “confirm” was no longer greyed out.