Please avoid sending a same query in different mediums. You may get duplicate response(s).
As svante responded the same query in forum Help and Support page.(https://www.axcrypt.net/forums/topic/file-name-is-not-an-axcrypt-file/) like,
Please provide more information, preferably screen shots of the situation so we can determine what the situation is. The most likely reason for this error is that the file is empty (in which case it’s likely that you are using the obsolete version 1.x of AxCrypt that is unsupported) or that you have renamed a file that is not encrypted with AxCrypt to end with .axx.
But start by providing more details.