Can Soul Exchange not tribute a face down monster?

since the oppenents card(s) is face down, the player using soul exchange doesn't know the level of the face-down monster and cannot tribute it. with Des Gardius if you don't have the masked monster neccesary for the tribute on your side of the field, gardius can't be summoned but LOL at the people who didn't know exchange can be used with ritual summons

Those are considered special summons (ritual is a type of special summon and masked beast card says that it can only be special summoned). Soul Exchange is exclusively for tribute summons.

As prior comment says and also you can't use for some monster, for example, big shield gardna (once)

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Ritual summon needs to know die monster level
Masked Beast.. dont´t know the problem maybe you didn´t have 1 of the 2 needed cards