caron symbol won't appear over upper or lower case letters. help please

If you press and hold the key down, do you get choices?

MtD

When holding a button with vowel, a popup list of characters does not have a caron characters like ǒ. How to add to the Accent Menu?

AAdding stuff to the character picker is pretty complex. The usual way to make Caron is to activate the abc extended keyboard and type option v and then the base letter.