Are you trying to check with the phone app, where in the past you have used the website? If so, possible that your password includes symbols (you know, things like #@$!, the types of characters that start showing up more and more in threads about FP+ and
MDE these days). The phone app requires passwords be letters and numbers only. If you set up your Disney password back when they accepted characters, you may need to reset your password.
This was what happened to me:
I had read a lot of bad things about problems with the app, so I had decided I would use only the website to set everything up. Only after I had all my tickets, ADRs, FP+, etc. did I decide I should at least make certain I could view them on the app, so I could use the app when we finally go to the park. I installed the app and tried to sign in, and got a message that either the username or password was invalid. Small panic. Notice that app mentions something about using only letters and numbers, but ignore it because my password has always worked at home. Try again. Same Message. Slightly larger panic. Un-install and re-install app and try again. Same message. Major panic. Now wishing I was at home so I can look on website, as fearing that everything is lost. Get home, sign in, password accepted and everything is there. Relief. Log off from website and try app. Invalid. No longer panicked, but certainly confused. Call IT. IT confirms that app can't accept password with characters, which my current password has, so will need to reset password. I ask if will mess up my FP+ selections (I am paranoid about this, and went through a similar paranoia a few days later when adding Magical Express, but that is another story). He said "it shouldn't" (slight anxiety, as that isn't the same as "no"). I change password on website while he remains on line. I get e-mail confirmation immediately. Test new password on computer. Invalid. Anxiety. He says wait ten minutes, and stays on line. Try again. Invalid. Reset again. E-mail confirmation. Test. Fail. Wait ten. Test. Fail. IT suggests I hang up, give system time, try in twenty minutes, and call back if still not working, saying anyone there should be able to help (By the way, all through this IT was very courteous and sympathetic, but unable to say why new password not being accepted). Hang up. Wait thirty (instead of twenty, to prove t myself I could). Try. Fail. Call IT. She (not rapid surgery - just a new person, and also friendly throughout call) conforms e-mail and password. Says looks correct. Says try again. Try. Fail. Says maybe system just slower to register changes than usual. She says someone else will need to work on my problem, and needs to ask lots of questions and then someone will get back to me (don't recall now all the questions, but I think things like type of computer, type of internet connection, web browser, etc.). While she is asking and I am answering, I keep trying password over an over. Just when she is about done - password accepted! I tell her. She says confirm everything. Tickets, ADRs, FP all there! She says something along the lines of, "well, the system must be taking a lot longer than usual in getting around to accepting changes today; I'll put a note in the system so others in IT know in case we have similar calls".
I know that was a long and unnecessary story, as my main point was the no-characters-in-password suggestion at the start of my post, but I thought some of you would want to vicariously experience some anxiety through me!