This can occur for a couple different reasons, the most common being that the merchant account does not have it's KYC documents and/or payout accounts on file and verified. There are imperative to have a functioning profile and must be completed before a successful invoice can be created.
There may also be a limit of your invoice volume which will prevent invoice creation. Merchants that have a payout account on file but no KYC, have a limit of $100 USD. This means that any invoice that surpasses $100 usd in price will fail. When a merchant has verified KYC on file and an active payout account, the invoice limit will be set to $10k.