Are you trying to use www.786securitysol.com9 for your Convertkit landing page? If yes, please make sure to add www.786securitysol.com9 under the domain section of your Convertkit account first.
Kindly go to SETTINGS>> DOMAIN>> ADD CUSTOM DOMAIN
Once done, it will give you a set of records you need to add to your domain's DNS. It would show "verified" once you add the records correctly.
If you have further questions, I recommend you email email@example.com to take a look at your account.