I wish I could take it back.
or
I wish I had taken it back.
Both are correct.
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htrekI wish I could take it back.
You don't want the item, but the person you got it from will not accept a return.
htrekI wish I had taken it back.
You have the item. At first you liked it, and the time period for returns is over. Now you don't like it. You wish you had returned the item before the expiration date