Translation:I have bought an armchair, but I don't know where I am going to put it yet.
«Куда его поставлю» is correct too. Sorry, I don't know why imperfective is used either, but it does sound natural!
My guess is that it's like [in annoyingly soothing female robot voice] ...installation in progress, please stand by... 25%... 50%... installation in progress, please stand by... 75%... installation complete; enjoy your armchair.
Sounds ridiculous with an armchair, but if it was something bulky and heavy enough to require several people to carry it and fragile enough for a mistake to be possibly fatal, then the putting becomes a separate important process, which is reflected in the language.
Just a wild guess.
Anyway, I looked in here to see if there is a discussed reason for I've bought an armchair but I don't know yet where to put it not being accepted. I see that there isn't.