Can the neutral wire be connected to the ground wire at the neutral bar in the breaker panel?

That connection has to only happen ONCE in the service drop. It usually happens at the first disconnect.

Any panel beyond that the neutral and ground must remain separate.

Ground rods are required for panels in detached structures.

Can the neutral wire be connected to the ground wire at the neutral bar in the breaker panel?
If you are asking if the neutral and ground can occupy the same termination the answer is no. Each neutral must be in it's own termination.

The neutral wires and the ground wires go to the same lugs in the panel,but not under the same lug ( there all connected to each other) unless your talking about a sub panel ( another panel that's fed off the main, like a detached garage panel) then they all must be segregated with no connection between them.