If you ever receive an error message within Kolleno, take a moment to ensure that you have selected all relevant fields.
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In the below example, the error message "Your request could not be processed. Please contact the support." can be easily avoided by selecting the Transactions: field above the note.

You'll be able to select the relevant invoice(s) to allocate to Promise to Pay.

You can add any notes applicable and click Save.

You'll then be able to see the Promise to Pay within the customer Timeline.