A discard can only be called to complete a set of 3 or more identical tiles (not for pairs or single tiles), unless it's the last tile you need for Mah Jongg.
Keep in mind that "NEWS" and "2023" (or other year groupings) are considered 4 single tiles, so you can't call a discard for that grouping (unless you're one tile away from Mah Jongg).
For more information on this rule (and other rules), check out this tutorial.