There are many reasons you may no longer be able to access the substitution, none of which can be resolved by action of anyone here. Only advice on actions you or the other person can take will be given.
As you're a new member to posting in the forum, you can't post links. But, can you at least tell us the message you receive.
Do you see the link on your substitution screen to click on to access the sub? How long has the player been on the world? How long has the substitution been in operation? (Note, the counting is of days since it was first accepted and can be out by a considerable amount if repeated send/cancel sub method is used).
Finally, which world is this on?