Can you use your credit card at Sesreim?
In short yes ... BUT ....
All the main places including the parks office (for your permit), most
accommodation (best to prebook and pay for accommodation in advance) fuel
stations and shops accept credit/debit cards BUT they are reliant on
electricity and internet both of which can occasionally be tricky in such
a remote location, so you must have a backup stash of cash (South
African Rand or Namibian Dollars are accepted).
Its your responsibility to be able to pay the bill so if you dont have sufficient cash on hand check with the staff that the system is working before ordering a meal, tanking up with fuel or shopping.