Why I can't I add contacts on my Telegram?

I can't add more contacts to my Telegram account. They said "[Name] is not on Telegram yet, would you like to invite them to join?" but the contact is indeed already existed on Telegram. I change my phone already and the setting was good but still the same I can't add anybody on my Telegram. So I just want to know if there are any solutions and suggestions that I could do to deal with this. Please help me.