Okay, first thing here :

A Tomb World is a planet type with specific habitability parameters, generally pretty low, which cannot be colonized by most species, unless you want to suffer tremendous penalties.

A Metropolis is a Colony designation which provides bonuses to ressource production.When you look at the planet management screen, it's the small window top left. The designation is picked automatically in this mod, depending on the buildings you chose to build in the first place. You can't set a Planet to a Metropolis designation freely.

They're not the same type of concepts. You can't change a Tomb World into a Metropolis, because it doesn't work like this. You need to terraform the planet first into a world that suits your species habitability needs, then wait for the process to complete (5k energy, 10 years).

You need 2 techs for this :
-Terraform (pretty self explanatory)
-Ecological restoration (allows you to restore Tomb World).

These two are mandatory to terraform Tomb World to a Planet type of your chosing, which can then be exploited to receive a specific designation (Metropolis, Rural and so on).

cheers