Go to Your Mautic Account dashboard, Click on the gear icon next to the username on top right corner. Click on Configuration settings >> Api settings and enable the Api.
Then go to "Api Credentials" >> create a new oAuth 2 credentials with a redirect url.
Redirect url should look like https://example.com/wp-admin/?ff_mautic_auth=1
(Your site dashboard url with this slug /?ff_mautic_auth=1)