Elias here with Shopify Support. Thank you for reaching out.
I understand you’re unable to install Mega Menu on the Minimal theme; however, you were able to do this for your other store using the Debut theme. Just to clarify, are you using a third-party app for Mega Menu? If so, which app are you using?
The reason I’m asking is because the Minimal theme doesn’t come with Mega Menu, so it’s likely that you’re using a Mega Menu apps from a third-party developer. It’s worth noting that the developer is always available to assist you, and you can reach out to them directly through the contact information found in the Support section of their app’s page. It’s also possible that the developer has a solution for the Minimal theme already, so I’d recommend reaching out to the developer as the first step.