Domain forwarding means your domain redirects to your actual domain.
So if you are using email lisa@getreplies.tech, then getreplies.tech should redirect to getreplies.ai
Where do I set domain forwarding?
If your domain is not redirecting, then ensure you enable this option in your domain providers portal. This is where you purchased or manage your domain (example: GoDaddy, Namecheap, etc.)
If you are using your primary domain
The domain forwarding check will show error if you are using your primary domain. You can ignore the error and continue to use the mailbox in the campaign. However, its not a good practice to use your primary domain and mailboxes for your campaign. For more information regarding this, refer the article Connecting your email accounts.