Here are some things to try:
1 close BlueStacks, restart device, open BlueStacks, try to purchase
2 in BlueStacks, log out of your account at Google store, log in again, purchase
3 try downloading another app from the Google store (to verify the store works)
4 contact support at BlueStacks for any suggestions
5 if you have other Windows devices, are you able to easily try on another? (tablet?)

It is the latest version of BlueStacks?