Already-created PO cannot be converted into a release order

What causes this issue?

You have attempted to reference a contract from a purchase order that
is not a <DS:GLOS.3526BFF7AFAB52B9E10000009B38F974>release order</>.
This function is only possible if you have selected the purchase order
with reference to a contract from the initial screen.

Error message extract from SAP system. Copyright SAP SE.