"She is weird, but I like her."
Translation:Ona jest dziwna, ale ją lubię.
Putting "ją" at the end of the sentence is not the best choice, but it was accepted with "ale" already. And "jednak" works fine to me in this sentence. So, added.
"Ona jest dziwna, ale ja lubię nią." Can you please explain, why am I wrong?
'nią' is Instrumental, you need Accusative. Instrumental and Locative forms of on/ona/ono start with n-.
For other cases, there are some n- forms but they are used only after prepositions.
And then, I wouldn't recommend putting "ją" at the end, as I wrote before.