That it is nonlinear is not pertinent. Although solve will surely fail to produce a result. Try vpasolve.
The reason for your error is that you need to learn to use tools like solve and vpasolve. Instead of = in those expressions at the ends, you need to use the == operator, like this:
There is a difference.
Again though, even once you fix the above errors, solve will still surely fail to return any result. In fact, it will probably run forever. Then you will ask another question about why solve does not work here. So use vpasolve, to at least have a chance.