"I do not remember what I have done."
Translation:Δεν θυμάμαι τι έχω κάνει.
Because it's "what I have done", not "what I do".
Or if you were expecting τι έχω κάνω: because it's "what I have done", not "what I have do".
With έχω as a helping verb to form the perfect, you need the απαρέμφατο form of the meaning verb, which looks like the third person singular (he/she/it) aorist subjunctive (the form used after να or θα). "He will do" is θα κάνει and so κάνει is the form you use in perfect and pluperfect statements.
With the comma, it is translated as "what have I done, I don't remember" which is not the same syntax at all. Without the comma τι έχω κάνει δεν θυμάμαι It is correct. It is one of the many many cases that in greek the syntax is flexible and the words can be moved around without changing the meaning. Because it would be impossible to accept all possible syntaxes in all sentences, it is better to stay close to the original sentence's syntax to make sure that those answer is accepted.
It is correct. I've added it, but, as troll1995 has mentioned above, the syntax of Greek is very flexible and it's not always possible/meaningful to include every word order imaginable because that would lead to exercises having a huge amount of alternate sentences, especially if that was done for every sentence in the course. As with other exercises, this one, too, is correct with the pronoun at the end of the sentence and it's a matter of emphasis.