If you are testing StaffTraveler through TestFlight, credit purchases are turned off. Tapping 'Buy credits' shows a notice instead of the usual packs.
How to buy credits anyway
Install the regular StaffTraveler app from the App Store and buy your credits there. Tap 'Open App Store' in the notice to go straight to it.
You can keep the beta installed alongside the regular app. Both sign in to the same account, and your credits, requests, and history are shared, so credits you buy in the App Store version are there in the beta too.
Why purchases are off in the beta
Beta builds are for trying out new features before release. Keeping real purchases out of them avoids charging you inside a version that is still being tested.
Everything else works normally
You can still post requests, answer them, earn credits, and collect Points in the beta exactly as usual. Only buying is unavailable.