What it means
'Invalid account' means the platform could not log you in with the login number, password, and server you entered. Usually one of those three is wrong, or the account is on a different server or platform version than the one you are using.
Common causes
A mistyped login number or password (passwords are case-sensitive). The wrong trading server selected. Using MT4 credentials in an MT5 terminal or vice versa. A demo account that has expired. An account that is not yet activated, is archived, or was reset by the broker. Or copy-pasting a password with a stray space.
Step-by-step fix
Re-enter the login number and password carefully, watching for case and spaces. Select the exact server name your broker provided from the drop-down — if it is missing, add it or 'scan' for it. Make sure you are using the right terminal: MT4 credentials work only in MT4, MT5 only in MT5.
If it still fails, the account may be expired or inactive — ask the broker to confirm the account status and resend your correct login details and server.
How to prevent it
Store your login number, password, and exact server name together. Keep demo expiry dates in mind. Install the platform version (MT4 or MT5) that matches your account.
Broker's fault or yours?
Most 'invalid account' errors are a typo or wrong server on your side. If your details are definitely correct and the account still will not connect, the broker may have archived or disabled it — worth confirming why, especially if you were not warned.