Once you turn on this setting, it will be enabled for your account across all platforms. Here’s how to set the reminder:
Select Account menu (your profile photo in the top left of the timeline view).
Select Settings and privacy.
Select Accessibility, display, and languages.
Select Accessibility.
In the Media section, select the toggle for Receive image description reminder.
Now, the next time you try to post a post with one or more images that are missing a description, you’ll get the reminder below.
When you select Add a description, the image description field will open and you’ll be able to write your description. If you select Not this time, your post will post without an image description.