
The message means that someone has entered your phone number when registering on WhatsApp. If it was you and you are registering WhatsApp on another mobile, it is normal. If it wasn’t you, it may have been an oversight on the part of a person or an attempt to steal your account.
The good news is that in order to use the account you need to verify that the phone number belongs to you through a code that will be sent to you. This is why it is vital that do not share the verification code with anyone. A classic way to steal WhatsApp accounts is through strangers who write to you on WhatsApp and tell you that they have

You will also know it by SMS, which is different when your account is being registered on another mobile
Even if you do not have WhatsApp open and active at the time, you will also find out about these attempts to verify your phone number through the verification SMS that reaches your mobile. The normal WhatsApp verification SMS is limited to showing you the code and little else, but when WhatsApp is in use on another mobile, the code warns you that “your WhatsApp account is being registered on a new device”.
What to do if you receive this notice
As WhatsApp explains in its help section, no one can register your account without having the verification code, so you should not worry. All you have to do is ignore the message and continue using WhatsApp as usual. without sharing the password that you received by SMS with anyone.
If in addition to the message someone writes or calls you telling you some little story that he needs you to send him the code, that person is trying to register your WhatsApp account for some reason. Keep in mind that even if you drop, give them the code and they register your account, they will not be able to access the previous messages. You will be able to recover the account with a new verification, but it may take a few hours before WhatsApp allows you to recover your account. If no one asks for the code and it only happened once, it may just have been a mistake.
How can you protect your account?
If you get a lot of notices If someone is trying to register your phone number on WhatsApp, the bad news is that there is not much you can do about it. Even if you activate the two-step verification of WhatsApp, it will not prevent you from receiving the notifications because WhatsApp asks for the verification code first and then the two-step verification code.
However, if you receive many warnings, it is highly recommended that you turn on two-step verification in your account. This way, even if someone somehow gets hold of the first verification code, they won’t be able to register the account because they won’t know this second verification code.

The second whatsapp two step verification code it is a PIN code that you choose manually and that you should never share with anyone. Optionally you can also enter your email to be able to recover the account if you forget the PIN, although this opens the door to other problems.
In short, the most important thing is that do not share any code with anyone and be especially careful if your SMS is accessible from another place (for example, in the web version of the Google Messages app) or as a notification on a smartwatch. As for the warning messages, nothing you can do: they will continue to come to you no matter what you do.