There are usually two reasons:
Incorrect password. If it is indicated that your account is
1. invalid, it means you have entered an incorrect password, as shown on the right.
2. Incorrect selection of the server. If "Common error" is indicated at the lower right corner of MT4, it
means you have selected the wrong server, as shown on the right.

When the transmission speed is shown, it means you have been successfully connected to the platform.