Mocking - cannot instantiate proxy class of property?

Create a mock object of UserManager first. Then setup its virtual method FindByIdAsync(given that the type of the property UserManager is a class AppUserManager and lets say this class implements IAppUserManager).

var yourMockOfUserManager = new Mock<IAppUserManager>();
yourMockOfUserManage.Setup(x=>x.FindByIdAsync(It.IsAny<string>())).Returns(() => null);

and finally

mockOwinManager.Setup(x => x.UserManager).Returns(() => yourMockOfUserManager.Object);