i can't add a gmail account why is it so?

I assume you mean that its won't go in by selecting Google in the account type. Have you gone into the Settings/Forwarding & POP/IMAP in Google webmail and made sure that either POP or IMAP are enabled? Have you tried choosing 'Other' as an option in account type when adding the account in Mail then inputting the servers manually?

Have you enabled two factor authentication? If not then you need to allow access for less secure Apps, see https://support.google.com/accounts/answer/6010255?hl=en Personally I would recommend the 2FA route. I use the option for a one time password in 1Password myself although you need to add a mobile phone number first before you can get to the option of using an authenticator App. It might sound a bit scary and a little daunting if you haven't done it before but actually it's quite easy to do.