If I knew how to edit my own comment I would but I cant find a way to do that.
Update:
Okay, I need to add more information to my post.
First, my Quicken data file is on an iMac desktop which still has El Capitan. For a decade I have successfully used Quicken 2007 from the iMac, my MacBook Pro with Sierra, my wife's MacBook Pro with Sierra and her iMac with Yosemite over my LAN (home network). That's four computers accessing the same data file, but never at the same time.
After updating my MacBook Pro to High Sierra and successfully recording a transaction from my MacBook Pro, I went back to my iMac and tried to open Quicken this morning. It reported that my data file was locked and I could not save changes. It would not let me in
So, apparently, opening Quicken from High Sierra somehow altered my data file so that it could not be used on a previous OS.
Fortunately, I was able to restore the data file from a backup. And it works fine again. I don't think I will be upgrading my iMac to High Sierra anytime soon.