You are on the right track, error messages should be concise and whenever possible suggest corrective action. The wording is misleading
"Maybe You should close the already open file before saving a new
As worded it gives the impression that the program has determined for a fact that the file is open, and suggesting politely that maybe the user should close it. If the file is not open elsewhere, and the problem is that there is insufficient disk space (for example) then as a user I would be irritated that the error message is suggesting that maybe I should close the already open file (which I know not to be the case).
What you are trying to say with "maybe" is that perhaps the file is already open (presumably by someone else or some other application), in which case it must be closed by the other program because it is an error to overwrite a file that another program has open.
This wording is clearer
Unable to save the document "/user/john/mydocument" - if the file is
open by another program please close it before attempting to save
From a usability perspective it is best if your program makes an effort to determine the cause: not enough disk space? no write permissions?
If your program doesn't check what the problem is, then the error message should indicate the uncertainty
Unable to save the document "/user/john/mydocument" - there may not be enough disk space, or you don't have permissions to write it, or the file may be open by another program - please close it before attempting to save again.