When I tried creating campaign via Ad API, getting account by id, getting funding instruments in sandbox mode, I got {:code=>“INTERNAL_ERROR”, :message=>“Internal Error”}.
Only one thing I can do in sandbox mode is get /accounts.
example:
mypc$ twurl -H ads-api-sandbox.twitter.com /0/accounts/gq10eu
{"errors":[{"code":"INTERNAL_ERROR","message":"Internal Error"}],"request":{"params":{}}}
best.