HOW-TO:
The error occurs when no payment gateway has been selected in the event set up (even though ‘pay online’ has been enabled for an interactive site). See the how-to text below, and the related FAQ at the link:
Why is Pay Online 'grayed out' (not available as an option) in payment page setup?
Setup Panel
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Select Event, then Preferences
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Choose External Connections tab
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Select E-Commerce Account
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Enter E-Commerce Reference
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Save
The E-Commerce Reference is the code (your chosen letters/numbers) you’re using to identify transactions relating to this event, for example in your payment gateway and on bank statement.