You are not doing anything wrong except for misreading the instructions. After the transition to CUBS, there is no way for the local unit to create subcategories under Ward Missionary. Those subcategories are created automatically by the Missionary Department for all missionaries actively serving from your unit (as noted here
For background, when the initial transition to MLS was made, there was a Missionary
category in MLS. At some point this was supposed to be phased out, and later became the Ward Missionary
category (as mentioned here
If after the transistion to CUBS, your MLS no longer shows a "Missionary" category, but does have a "Ward Missionary" category, this is to be expected.
If after the transistion to CUBS, your "Ward Missionary" category does not have ANY subcategories for the full-time missionaries currently serving from your unit, that's a different matter. This
thread might help with that.