Can't add Flex Relate field to Contracts module via Studio

I'm trying to create a Flex Relate field in the Contracts module but I don't even have the option to do so as it is not in the Data Type dropdown. Flex-Relate appears in all other modules, even backwards compatible ones. 

There are no other Flex Relate fields that already exist for the module. Is this a bug? Does anyone have a workaround? We are on-site so we can add some code if necessary. 

We run Enterprise 7.5.0.1

Thanks in advance!