Try moving the file to a different folder. Move the file out of the Programs Folder if on a Windows Machine.
Exact details are sometimes helpful.
OS? Version? FMP version?
Where exactly is the file stored, internal hard drive in a user's folder, external hard drive, shared folder, etc.
Have things changed since the last successful use? OS upgrade.
The file can be locked by the operating system, highlighting the file and "Get Info" or show Properties is a first step.
Exact wording of the error message, shown by OS or FMP?
Backup the file twice if it is not backed up. Make a test copy, try to open, move to another location, etc.
Didn't work. Also, I should've mentioned I'm on a Mac. I used the Recover option and the recovered version works.
OS & Version: Mac OSX El Capitan 10.11.4
FMP Version: Created using Pro 14.0.4, trying to open with 15 & 14
Stored in a shared folder
The only change since last used was upgrading FileMaker to 15. But the file won't open in 14 either.
It is not locked.
The message is FMP, it says "This action cannot be performed because this file is not modifiable."
Regarding your last suggestions, I tried to Recover and that seemed to work fine.
I have never used Mac so I'm not sure but if such a problem exists on Windows machine you can solve it if you will run the runtime as administrator. On windows its attribute/permissions problem.
I have a database that is stored locally that both me and a co-worker worked on for our company.
I tried to Recover and that seemed to work fine.
Assuming that means that the recovered copy was modifiable, this leaves me thinking that you located the file on a shared directory where you and your user could both directly access the file in order to open and work with it. If this is the case, this is a set up that risks damaging the file and is not a recommended way to share a solution between different users. The file should be, at the very least, hosted by one user and accessed by the other as a client of that hosted solution. The file should not ever be located in a shared folder for purposes of sharing between users.