When working in the Classic Editor you can not by default.
You will have to use some lines of custom CSS code (depending on your theme, layout etc).
As an alternative you can use one of the plenty available gallery plugins
(I would personally go with the custom css since it is a very simple thing you ask for)
Blocks (Gutenberg editor)
The problem arises because the site builder that is part of the Make theme requires the Classic editor to work, except for some posts for which it is able to use the Gutenberg (also known as the "block") editor.
Solutions therefore include finding a way to drop caps using the Classic editor while continuing to use the Make theme, and using a theme other ...