Troubleshooting Payment Options
New Payment Method Not Available on Invoices
If you setup a new payment processor than it's likely the client record needs to be updated to enable the new payment processor.
In the example below eWay was added after this client was created. After selecting the eWay option the payment button will show on the invoice page.
Disable Add-on
You can also disable the add-on that provides this feature if you don't need it.