If you have built a standalone app with syncing turned on, and you get an error something like "Failed to login to remote database - 802 - Unable to open file", it probably indicates your setup is not complete.
This can happen if you have not uploaded the gateway (interface) solution to FMS, or if any of the server settings (address, username or password) are incorrect. Check the lessons on setting up your gateway, uploading to the server, setting your fmrest permissions and make sure your Data API is turned on. Remember you must upload both the original FileMaker solution and the gateway (interface) solution to the server. Both must have passwords set, and they must match. It is also worth checking that the admin account for the original and interface gateway file matches. For example, if your account for the original file is using "myspecialadmin" as the username, and the interface file has defaulted to "admin" as the username, access will fail.
If all of this is set up correctly, check that your app is able to contact the server ie you are on a working network and no firewall is blocking access.