Where does “безуспех” come from?

безуспех (Serbo-Croatian) comes from Serbo-Croatian без-, from Proto-Slavic bez- — without; -less.

Ancestry of “безуспех”, step by step

Step	Language	Word	Meaning
1	Serbo-Croatian	без-	dis-, un-; -less, prefix appended to adjectives...
2	Proto-Slavic	bez-	without; -less

безуспех — Etymology, Origin & Word History | Etymology Explorer