3/4" compression coupling leaking for alpine burnham U.S. boiler
If it is leaking at the point I highlighted in the photo below.  The drain stub is most likely not a 3/4 PVC pipe or is designed for a different type of coupler.  You would need to check that it is a smooth 3/4 PVC pipe exiting the boiler or is it a barb that requires a push on and locking clamp.
Just to clarify where the leak is and ask for more details on where the leak is.  See questions and requests in the picture below:

Edited to add additional information:
From your new pictures, it appears that the original drainpipe coming out of the boiler has been modified and is no longer a 3/4&quot; PVC pipe. See diagram from manual below:

Your current connection as shown in your third picture appears to be a soft connection hose.  You would have to open the front panel and see how that clear hose is connected to the trap.  You might be able to connect a 3/4&quot; PVC pipe in the trap area come out the outlet hole and match up to your Compression coupler.
A view of the pipe inside the trap area might help us provide a better answer on how to repair the leak.
You at least know why it is leaking.

Added details on how to fix the leak or make a connection.  
As stated earlier, the draintrap pipe and validated by your new picture indicates that the drainpipe is a soft push-on hose.  From your picture of the trap in the boiler, the type of hose clamps used for connections (marked in the picture) shows that the trap and piping is a soft but stiff hose.  A compression coupler will not be able to assert enough pressure on the hose to prevent a leak.


Below is one way to provide a better connection to the soft hose from the boiler.

Insert a short ½” PVC pipe into the hose from the boiler and secure with a wire or ring clamp.
Attach a  ¾”-1/2” PVC reducing couple to the end of the ½” pipe
Attach a short ¾” pipe to the other end of the reducing coupler (long enough to reach the union.
Attach the other end of the union to the existing ¾” PVC pipe.