When coming across 'No Mandate Found' this means one of two things.
1. The customer has an active mandate but is not yet linked to their unique Xero ID.
2. The customer has not yet signed their Mandate.
If you go to the Invoices page to take a payment and the customer has 'No Mandate Found' against it, go and view the customer from the Customers tab. The information on this page will indicate which of the above reasons is stopping you from take payment.
If the customer shows as having an active Mandate but is not yet linked to Xero, you need to go to the 'Match Customer' page and match them up. You can then go back to Invoices and process the payment.
If the customer shows as being linked to Xero but no active Mandate, they have not yet signed. You can send your customer an email reminder from the Customers Page using the 'Actions' button.
The 'Mandate Status' and 'Linked to Xero' columns show when the record has a blue 'tick'.
If you see a red 'cross' in the Mandate Status column this means that the Mandate has been cancelled.