I would probably call it a legal act (though I would need to research it a bit more to be sure) and probably just use "enforceable".

...får en rättshandling, som annars är att anse som giltig, inte göras gällande : I agree in that construction with 'may not be invoked' or *is unenforceable* as a standard Anglo-Am. contract law term.

med vetskap om dem åberopa rättshandlingen och den gentemot vilken rättshandlingen *företogs* - IMO taking the term out of the realms of a physical deed or doc. (with *notice* / knowledge of those relying on the legal dealing or transaction and in relation to whom / vis-à-vis whom the legal dealing or transaction had been conducted).

åberopa overlapping in part with göra...gällande: invoke, assert, claim, enforce, maintain, establish, *plead / aver*, bring forward (as evidence), etc. etc. Gullberg. Averment in civil procedure is an allegation or assertion. Otherwise, 'set up' a legal defenc/se.

rättshandling = legal act (*transaction*), 'act involving legal consequences'; court proceeding/s or procedure, Gullberg.

Note that the dictionary-undriven term of legal dealing in English often covers a multitude of 'sins', to wit: bank operations and transactrions.