If the person is living and you want to "Add New Memory Source" like a birth certificate you will not find the command. WHY?
This command only shows up for deceased people.
There are 2 options to add a memory source to a living person’s ID.
Use the Family Tree mobile app. This option has existed for some time.
Use the newly enhanced Source Box to add a memory source to any ID you can see, living or deceased.
This blog article has some information:
Just be aware that some of your records may never be seen. If you become deceased before the people associated with the living records you created, FamilySearch does not have a process to release those records. Here is interesting reading: https://yanceyfamilygenealogy.org/when_you_pass_away.htm