どそりゃ

[mella]

だけどそりゃ間違いだ
You were only wrong.
どそりゃ I don't understand this word.

[Hyperworm]

Parsing error!
(So often this is the problem...)

だけど(however) + そりゃ（＝それは）

だけど is だ(＝です) + けど. See also だが. Basically means the same thing as しかし.

[micahcowan]

So, given what Hyperworm has pointed out, a better translation (without knowing anything further about the context) might be, "But that's a mistake." だけど but; そりゃ that; 間違い a mistake; だ is/(copula). Other translations are possible, but "You were only wrong" is not one of the possibilities.