When you encounter this kind of error, it usually takes at least an hour to resolve.

Looking at their design, they intend to use their own interface to broadcast all on-chain transactions. But after going through the code, there’s no requirement for users to sign directly with their wallet (probably to prevent bypassing them through private RPCs). By this logic, the platform should be covering the transaction fees, right? But in reality, the fee payer is still set to the user's address.

This is awkward—the user hasn't signed, so how can they authorize the fee deduction? The transaction just can't be sent out.