ygolonac(core set 122) has the folowing texbox: Action: Pay 1 to choose and ready a character. That character must commit to the same story as Y'Golonac, if able. can you use it on your opponent's redy characters to force them to come to the same story as ygolonac?

Even though you can't 'ready' an already ready character, the separate, second part of Y'Golonac's ability does work to force that character to defend, as per the Frequently Asked Questions section in the FAQ (available for download on the Call of Cthulhu Support page) --

Q. If Y’Golonac (Core Set F122) which reads: “ ... Action: Pay 1 to choose and ready a character. That character must commit to the same story as Y’Golonac, if able. ” uses his ability during the Story Phase after both attackers and defenders have committed their characters to stories, does the effect force the targeted character to commit to the same story as Y’Golonac?

A. No. Y’Golonac’s ability does not create an additional window through which characters can be committed to stories. If anything (including game effects) prevents a player from fulfilling the entire effect of an “if able” clause, that effect is ignored.

Also, it is important to note that Y’Golonac’s ability to ready a character exists independently of his ability to force characters to commit to the same story as himself. Therefore, he is able to target a ready character with this ability, and even though the first part (ready a character) does not resolve, the second part of the effect does resolve as long as there is nothing preventing them from committing to the same story.