Only Ultimate members and Friends can send private messages to models. Regarding public chat, there are two reasons why you may not be able to use send messages there:

1) You’ve been banned temporarily for spamming.
2) The model has set her chat only for users with verified accounts or tokens.

We suggest you buy tokens and verify your account to be able to use any public chat you want. You can also contact support anytime to check your spamming status.