This is caused by a mismatch between the event currency and the currency set on the interactive (registration) site.
For example, if the event has been cloned and changed from one currency to another (in Event Preferences), the registration site would still be set to the previous event’s default currency.
This error can also occur if changing from a single to multi-currency event, or vice versa.
To fix the issue, ensure the event currency matches the interactive registration site currency.
You may need to re-add items to the registration site and/or re-save it, then clear your cache, to see the updates.
To see different event currency options on the interactive registration site, you need to first make the event a multi-currency event (in Event Preferences).
If needed, change the event back to a single currency once you’ve saved your changes to the registration site.