Not all CITs are showing up in the drop down when adding Investment Type under MUX - Administration - Attributes - Investment Parents
Release : 16.1.2
The Investment Mapping feature introduced in 16.1.2 is focused on Hierarchies and hence the CIT should be enabled for Hierarchies to show up when adding Investment Types.
Under the CIT Object Properties page, check the box "Include in Hierarchies"
https://techdocs.broadcom.com/us/en/ca-enterprise-software/business-management/clarity-project-and-portfolio-management-ppm-on-premise/16-1-2/release-information/clarity-ppm-release-notes.html#concept.dita_8ebed6ba1fc5de89d746baa8a9be14ef035caa38_InvestmentParentMapping