Why I am not able to login on two smartphones? What is the meaning that I an not able to login on another device?
I am sure that user and pwd are correct but on the second device it give me this message "user/password are not correct".

I try to set the smartphone with another account and it is easy to do that

I regularly use my account on the smartphone and on the pc
Having two phones logged into the same Google account works perfectly fine. If it tells you the username or password are incorrect, then most likely one of them is incorrect. In particular, if you're using different soft keyboards on the two phones, take extra care when typing punctuation characters and numbers: they could be in different places on different keyboards, so you could type the wrong one without realising it.