Reset Apple ID password using find my iPhone .
On your iPhone device , click on home scroll to the next page > click on extras > you will find find my iPhone click on it , enter your Apple ID only , click on forgot Apple ID or password , click on next.
Reset options - choose how would you like to reset your password ( a )
Click on reset through mail and click on ok .
Then open your rescue mail with web browser ( server account ) , open mail > click on reset password > two fields will be shown .
( a ) new password …..
( b ) verify password ….
Enter the credentials , and click on reset password , the password is changed .
If you click on - answer security questions - click on the right arrow .
Date of birth , verify your date of birth to continue > fill up the credentials > click on next and same pop up window will appear .
New password ….
Verify password ….
Fill up the credentials and click on reset password , the password is changed .
Either the mail address or password that you are trying to sign in is incorrect as there might be typing mistakes in both , enter the mail address and password correctly , sometimes if incorrect credentials are entered incorrectly multiple times server assumes some one is tricking and using phishy attempts .
Server doesn't know who is using the Mac , it assumes and locks the account see the complete details in the article If your Apple ID is locked or disabled - Apple Support
The second reason if Apple ID account doesn't exist on the iCloud and it is erased from the server How to delete your Apple ID account - Apple Support
And that can be verified using this link https://iforgot.apple.com/appleid#!§ion=appleid
Enter the correct credentials if the account exists a green check mark will always appear the account is verified .
Note : if you don't remember Apple ID and its password , and iPhone is in your possession better change the account to 2FA see the articles Two-factor authentication for Apple ID - Apple Support
Get a verification code and sign in with two-factor authentication - Apple Support
The secondary authentication can be only be opened with the help of apple support advisors Contact Apple for support and service - Apple Support
( a ) If you lose answers for Apple ID provide credit card details that was added in the account .
( b ) if you have lost Apple ID password provide invoice copy of Mac to apple support advisors .
( c ) Contact apple support , they will send a link on mail account , click on link after 24 hours to reset apple id password , however the link is valid for additional 3 hours .
( d ) Support pin is used by apple support advisors to verify the account when you know apple id password Generate a temporary Support PIN - Apple Support