FileMaker 13 and earlier versions of FileMaker have not been tested or certified with OS X El Capitan. These products were developed and released before OS X El Capitan became available.
While these earlier versions may install and run, you may encounter installation and stability issues for which there is no resolution
As of September 23, 2016, FileMaker 12 products are no longer available
If you are new to FileMaker, you started on the wrong version. I would suggest upgrading for the best compatibility. In the meantime, any other update to your OS or another program could have broken filemaker. If a restart of the computer doesn't fix it, then most likely you need to try reinstalling or upgrading.
Is the file you are trying to open a local file on your machine, or something hosted on another server? If it's remote, then you should contact the host. If you are the host and it's not local, you need to troubleshoot your host machine.
The company I just started working for has been using this version for awhile now.
Any tips on how to upgrade from FM 12 to a better version. Thanks for the help and reply.
Get your company to purchase a new license, then upgrade. I'd recommend an annual license so that you just automatically get the new versions as they roll out every year,
I just upgraded to Sierra a few weeks back. I had FM13, 14, 15 all running on my system prior. Upgraded to Sierra so I could use FM16. I didn't care about the past versions have when I did click on FM13 it gave me a message that an older version of java (I think that was it) was needed.
I did not try that but it might be worth a try to keep you going for a bit.
FM is committed to a new version each year so you might want to consider that in your purchasing decision.