Hey,

I bought AoE 2 DE, but I can't get WK to install on my pc. How do I fix this?

You cannot play WK if you own Aoe2DE. You must own HD with all DLCs, if you want to play WK.
The developers of WK however may release a WK for DE in future. There is no ETA yet.

Thanks for the answer. That's unfortunate, I'd like to play a bit on voobly but it's going to be too expensive that way. I'll wait, I suppose.

You can still play Age of Conquerors if you own the old game. You don't need WK to play on Voobly.