How to solve the issue when the merge page is not loading
In some Salesforce environments, users are protected against "clickjacking". As a result, it's possible that the merge page will not load. This can be solved by whitelisting your domain. Follow these steps to resolve this issue.
Whitelist your domain
| 1. | Navigate to the Salesforce Setup - Security - Session Settings |
| 2. | In the page, look for "Whitelisted Domains for Visualforce Inline Frames". |
| 3. | Click "Add Domain". |
| 4. | at "Domain", fill in your ORG's domain.  |
| 5. | Click Save. |
That's it, you're all set. The merge page will now be able to load in the modal.