Some payment methods might take up to 72 hours to be confirmed on Xsolla's system. If you are using an instant method such as credit card and the problem persists, you must talk to Xsolla's support about it.
To do so, go to https://help.xsolla.com/ or open the Gift Shop in game, click on "Get More Reward Points" and click on "BUY" on any RP amount. On the next page, scroll down to see Xsolla's support icon:
If contacting Xsolla does not solve your problem, contact our support on Support Request > Reward Points Issue with a screenshot of their answer.