"Тока" is a misspelling of "только" which you frequently encounter because in fast speech, that "ль" doesn't get pronounced.

Sometimes, it's deliberate; the writer is going for the effect of transcribing colloquial speech - like in English, you might write "innit" instead of "isn't it" to render a casual spoken conversation.

And sometimes, it's a sign of a illiteracy - like in English, when people write "should of" instead of "should have" because it sounds the same and they don't know any better.