you have to activate "Variant Grid entry" feature in purchase settings:
then Go to purchase RFQ -> Create -> Add product line (choose one with dynamic variant) -> choose the right combination for you -> add -> confirm RFQ => go back to your products => the product variant will be created
Hope this will help.
Thank you Alouna, you are correct once again. Forgot that I changed the language in profile settings and not all products had variants in that language ...