If under syncing the syncing method is fixed at don't sync this normally means the data set is a secondary computer data set backup that has been restored using RESTORE FROM BACKUP.
At any time have you been syncing with another computer?
If so there is a way to convert a secondary to a primary data set so the syncing method can be changed.
As you have not been syncing with another computer and restored a secondary backup, I don’t know is the following is appropriate for your issue.
However, use FILE – EXPORT BACKUP is case something goes wrong to make a backup then:
Attached is a file "md2017_convert_secondary_to_master.py".
To use the .py file, first save it to your computer, somewhere you can easily access it like your downloads folder or your desktop. Then follow these steps:
1. In Moneydance, select Window --> Python Scripting.
2. In the window that opens, select 'Open Script' then navigate to the .py file wherever you saved it on your system.
3. Select 'Run' to execute the file.
4. Wait a few moments until you see a message on screen, confirming the script has run successfully.
5. Close and reopen Moneydance.
Click FILE - SYNCING, and you should now be able to select the required sync method. The best and most flexible option is called “Dropbox Folder”.
EDIT Thanks to dwg for spotting I originally posted references to 2015.
You are running an old build of 2017, you should update the last build this fixes a number of bugs.