I can see the reason for "you need reputation to upvote". But I should be able to upvote answers to my question, since there could be more than one answers that work but only one that can be accepted.
You need 15 reputation points to upvote, even for answers on your own question. It isn't that hard to get 15 points: 3 upvotes on one or more question will do it, or 2 upvotes plus 2 questions where you accept an answer.
Once you reach 15 points, you can come back to your questions and upvote answers.
It is by design that voting, even on answers to your own questions (or to questions that you've answered), requires a small amount of positive participation. Before you can start formally giving your opinion on other posts (by voting), you need to make a small recognized contribution to the community. This requirement also makes some forms of vote fraud more difficult (if you attempt to make multiple accounts in order for them to vote for each other).