Thanks for letting us know here about your experience and difficulty accessing the payment system in the Galaxy Store and Galaxy Theme so you can design and change your beloved Samsung Galaxy phone's app icons.
If you have a physical GCash MasterCard, you can use that to pay on Galaxy Store, and try any other approved Credit Cards.
If that doesn't fix the problem, you may submit an error report through the Samsung Members app as follows:
Step 1. Long press Samsung Members icon on Apps screen and tap Error reports.
Step 2. Tap OK on agreement popup to send system log data.
Step 3. Select Symptom category "Application".
Step 4. Describe the problem in details and SEND.
Should you need further assistance and guidance, you may contact us via our online Samsung Customer Care support channels below.