To assign your operative a uniform for the Uniformed Access ability:

Step 1: Assign Spawn Model

Open Operative > Outfits > View/Edit Spawn Model and set the Spawn Model to your desired uniform type e.g. Uniform.PMC_Captain_Elite. Click Assign to Operative when done.

Step 2: Add uniform to operative's outfits list

Go to Operative > Outfits > View / Edit Outfits and add the same uniform you selected above to your operative's wardrobe.

Using the above example:
For female: Uniform_PMC.unif_military_pmc_captain_elite_f_01
For male: Uniform_PMC.unif_military_pmc_captain_elite_m_01

If your selected uniform has multiple variants (_01, _02, etc.), you need to add them all. One variant will be randomly selected and saved.

Step 3: Add the Uniformed Access perk to your operative

Go to Operative > NPC Data > Perks and add the Access_Albion perk. (Or Access_Construction, Access_Medical, etc. depending on the type of uniform)
Save and reload your game. Now you should be able to toggle your operative's uniform with the Uniformed Access ability.