The water heater in my house 4500 watts @ 230v. I am replacing it with a Titan tank less water heater model N-120, which is rated at 11 KW @ 230 V. My main beaker is 150 amps, the panel is rated at 200 amps. Any problems with the main breaker?

The only thing I question is whether an 11kW heater will keep up. Most whole house units are far bigger.

along with Stan's comments, you may be ok as the load for 11kW @ 240 is 46 amps.
Now, what you don't know is the load of everything else...
If you are already at 80% load you may be overloading your service...

If you have 150 main, the panel may be rated for 200 but you won't get more than 150. That's a lot of draw for 150 Amps.

Why? Electric tankless heaters are energy hogs, not savers. And as stanfortyman said, it may not be enough. An 11K heater is half of what most tankless heaters are for residential use.