Two sided printing issue

[ChrisMSPT]

I am on a Mac using Catalina. I am using Pages

I tried to print using "Two-Sided" option.

I got the following message when I clicked the "Two-Sided" box:

The Two-Sided option, Long-edge binding, is not available when the destination paper size is set to Index card Letter 8.5x11in..

To scale to fit to US Letter, click Continue. To change Two-Sided to its previous setting, click Cancel.

I went to "Fille" --> "Page Setup"  and selected "Paper Size", it had it set as "Index Card Letter 8.5x11in."

I changed it to "US Letter", and clicked "OK".

I went back to "Print" and clicked on "Two-Sided" again, it gave me the same error message.

I went back to "Page Setup", it was set again as "Index Card Letter 8.5x11in." (meaning that it had defaulted back to that setting).

Any advice?

[Repairatrooper]

Most likely the printer does not support duplex printing of index card, it is too stiff and the system will refuse to perform a function that might damage the printer. Try duplexing with plain paper and the settings set to plain paper instead of index card. If it works, then you know you cannot duplex index cards. If you are trying to print duplex on index card paper stock. Print one side and then refeed the paper.

[ChrisMSPT]

I am not printing with index cards.

I am printing with regular plain paper.

The problem is not with the paper, it is that when I change the setting from "Index Card Letter 8.5x11in." to "US Letter" IT DOES NOT STAY AT THAT SETTING, BUT REVERTS BACK TO "INDEX CARD"

So how do i get the setting to stay at "US Letter"?