Edit: Make sure that your pawn is wearing a |Pilot suit| before making them |Enter the maintenance gantry|

1. Make sure the module shelves are set to /Critical/
2. Once your colonists have carried the modules to its /module shelves/ (Make sure that all & optional parts are there)
3. LEFT click the /maintenance gantry/ then press the /exosuit/ button near its detailed tab
4. Once clicked the /exosuit/ button, RIGHT click the center window to show the selectable /core modules/
5. Same as 4! to all parts you apply, happy hunting!

colonist needs to wear a pilot suit

thank you so much bro, didn't know you needed to right click the center window, kept thinking I was doing something wrong(building wise) or had a mod conflict