It's either that you don't have at least 50 revival bottles to revive one ninja or you already revived ninjas and the ninjas you revived have died. You can revive max 3 ninjas. If it's neither, it might be a bug and I suggest you putting this post on the Bugs & Support section.

To clarify what Danzo said, you can revive a total of 3 times, it can be the same ninja 3 times or 3 different ninjas, but a total of 3 revives per masuri

Also, an additional possibility, since this was not explicitly excluded in the OP, if all your ninjas die, masuri ends, regardless of whether you have used revive or not. So if you are on your last team and intend to revive anything in the event that you lose, you should revive ONE of the ninjas before going into battle with your last team.

Hello~

Any particular errors? Also, if you could please report it here at the bugs section with a screenshot or details of the bug so that the team can check it. Thanks.

Edit: Oops, I replied a little late. If its not like what Danzo and PraiseLuka have said, then it's definitely worth checking.

Closing thread.