Hello Sarah,
Hope you are doing well. I am sorry to hear that you are experiencing issues with trying to connect OneDrive account with Acrobat Reader Mobile.
Just so that we can understand better, can you share a screen shot or a screen recording of the exact scenario that happens when you try to sign in to One Drive?
Also while you do so, once try these steps:
1. Log out of the application;
2. Restart the phone;
3. Log in back with the same e-mail address that has the active subscription.
Let me know if this helps.
Regards,
Souvik.