"Oh, the wrong museum."
Translation:Oho, väärä museo.
Väärä is an adjective and väärin is an adverb. That may not be very helpful because English uses adjectives and adverbs a little bit differently, but basically, before a noun, you can only use väärä.
väära museo = the wrong museum
After olla (to be), you can use either one, but the meaning is a bit different. Väärä refers to the wrong entity of something previously mentioned. Väärin indicates that something is incorrect in its content.
Rakennus on väära. = The building is the wrong one.
Vastaus on väärin. = The answer is wrong.