Chances are, you don't want to run the default TFS installation. You have two options:

  1. Delete /home/otsmanager/forgottenserver and put your datapack in the same place and the executable as /home/otsmanager/forgottenserver/build/tfs OR
  2. Reconfigure tfs.service to reflect your directory and executable file.

In order to go with the second option, you should edit /etc/systemd/system/tfs.service and set up working directory and binary path.

 1otsmanager@176-31-89-137:~$ sudo mcedit /etc/systemd/system/tfs.service

Change the following two lines to reflect your path and binary

 1ExecStart=
 2ExecStart=/home/otsmanager/ots/theforgottenserver  
 3WorkingDirectory=/home/otsmanager/ots

Save the file (F2) and exit mcedit (F10).

Now you need to reload systemd configuration:

 1otsmanager@176-31-89-137:~$ sudo systemctl daemon-reload

Now you can start the server in the regular way again:

 1otsmanager@176-31-89-137:~$ sudo systemctl start tfs