Payment for goods on the item page

The payment system works on the page of all products of the store, but does not work on the page of a separate item.
In the editor, during the preview, it shows that the system cannot find the ID of item and, because of this, does not display the payment page.
I have already tried to one member of our community to indicate what the error: “You need to use queries to query for desired item form database. And then assign that item to the repeater…”
To be honest, nothing is clear. Can someone explain more easily?

Hey there, you may want to contact customer care for assistance.

Yes, nobody answers there

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