To solve this problem you have to use bar which is at the very bottom of the feature manager. Pull it up and down without letting go of the button above the sketch that contains the symmetry action, add a sketch or modify an existing sketch. If the symmetry function is used in the sketch itself, it is in this sketch that the modification must be made.
You should see your building tree to tell you precisely what to do.
OK, so you can apply either method. The choice of one or the other depends on whether the functions exist (my method) or if you are going to create them (the @Zozo_mp method ).
Regarding the @Zozo_mp method , it is also possible to click on the function above which you want to insert and choose "resume":