What is happening
When you try to log in, the server your broker gave you is not in the list to choose from. Without the correct server selected, the login cannot succeed even with the right number and password.
Common causes
You installed a generic copy of MetaTrader rather than the one from your broker, so it does not include their servers. The server list has not refreshed. You typed the server name slightly differently from the official one. Or the broker recently changed or renamed its servers.
Step-by-step fix
The most reliable fix is to download MetaTrader directly from your broker — that build ships with their servers preloaded. Alternatively, in the login window choose to add a new broker/server and type the exact server address your broker provided, then let it scan and appear in the list.
Double-check the spelling of the server name against the broker's instructions; even a small difference stops it connecting.
How to prevent it
Always install the platform from your broker's own download link. Keep a note of the precise server name. If the broker announces a server change, follow their update instructions.
Broker's fault or yours?
Usually it is having the wrong or generic install on your side, easily fixed by using the broker's own version. If the broker cannot tell you a working server name at all, that is a basic failing worth taking seriously.