The error message states, the encrypted file which you are trying to open does not exist in the location. Means the file is not downloaded completed.
Just download the file from remote service to your mobile/tablet.
Now open the AxCrypt app and then open the file via the unlock icon.
Still, you are facing the same exception, please write a mail to firstname.lastname@example.org with detailed information like which device you are using? which operating system? AxCrypt app version? Does file name contain any special characters? and etc..