Both options are fine but the original isn’t. It’s clearer to say: Now, I can do something. (Starting with and emphasizing ‘Now’ suggests ‘now and not previously’ more clearly than putting it at the end.)

Or you can say ‘I couldn’t have done it before’

They mean the same thing, but the second has a little bit more emphasis in my opinion, as if it's amazing that you can now do it.

Both your examples are okay Pelin.