Neither. You already said "I." So: I cannot afford to buy that computer.

Answers are for North American English. I can’t afford that computer. (most natural) I can’t afford to buy that computer. (natural) I can’t afford to buy that computer for myself. (natural) I can’t afford to buy myself that computer. (natural) I can’t afford __MY___ buying that computer. (Correct, but somewhat unnatural in North America. “... my buying" a possessive gerund phrase.) I can’t afford _____ buying that computer. (Correct, but somewhat uncommon in North American English. “... buying ..." is a gerund phrase) I can’t afford ____ buying that computer for myself. (Correct, but somewhat uncommon in North American English. “... buying ..." is a gerund phrase) I can’t afford ____ buying myself that computer. (Correct, but somewhat uncommon in North American English. “... buying ..." is a gerund phrase.) I can’t afford __MYSELF__ buying that computer. (Wrong in North American English. I can’t comment on British or Australian English.)

Hi Olena, Miguel is right about the sentence you have written. But maybe you want to say something more like “I can’t afford to buy that computer myself”, where the emphasis is that you want or need someone else’s help/money to buy it?

Depends on the full sentence. If you’re the subject and object use “myself”.

myself