I am trying to install a portable hot tub in my back yard. Starting at the tub, I ran two hots and a neutral plus ground back to the 60 amp GFCI in my disconnect box. Then, I ran the same wires back to my main panel, but my main house panel does not have a neutral bar, what can I do?
Connect the white wire to the bar the other white wires are connected to -- it is not rare for a main service panel to have a combined neutral and ground busbar, as that is the (only!) point where neutral and ground connect.
Neutral and ground are kept separate absolutely everywhere, except one place: the main panel. Here, it is OK to simply use one bus for both neutral and ground.
You will probably find a lot of whites and a lot of bare wires going to the same bus. That's the one.
Just for reference, DON'T do this in a sub-panel or you can create a lethal hazard.