Which do you have?
An Admin account, or a standard account?
A standard user that is allowed in Users & Groups pane, to administer the computer, means that you don't have a standard account. The Users & Groups pane will show that your account is an Admin account.
Check your Users & Groups pane, in the System Preferences. The accounts are listed there, along with the type of account.
If you are a Standard account, then that's why.
You have to be logged in to an Admin account before you can use a sudo command in the terminal.
A standard user cannot perform a sudo command.
If you DO have an Admin user, and it is your current user (you are logged in to that Admin account), and it still won't accept your password, then likely you are not using the correct password.
Also, don't forget that you won't see the password as you type it in, as the terminal does not show your password in any way, not even a row of * * * * *
You type the password without seeing anything as you type, then press enter.
There is another method that allows you to do a sudo command.
Type the command su AdminUserName, then press enter. (Replace AdminUserName with your actual admin user name.) You will then enter the password for that admin user, and a NEW window will open showing the admin user in the prompt (not your standard user name), and enter your sudo command. It will ask for your password (for that admin user), and the sudo command should run.