Hello andrés296,
If the account is not be added, the status shows "Checking", and you're unable to send or receive email from the account, this may be due to the settings used to set the account up. You may be able to confirm the appropriate settings for the account by entering your email address in the "Mail Settings Lookup" field here: Mail - Official Apple Support
If the settings for your account aren't available there, you'll want to contact your email service provider to confirm those settings.
If your settings need to be changed, you can edit them by following the steps in Change Server Settings in Mail on Mac.
"To change these settings in the Mail app
on your Mac, choose Mail > Settings, then click Accounts. Select an account on the left, then click Server Settings on the right.
Note: The settings shown vary depending on your account type and whether the account is enabled."
Regards.