Where did TRAPPIST-1a go?
As pointed out by @Adwaenyth this really is comprehensively covered in https://en.wikipedia.org/wiki/Exoplanet_naming_convention
The convention for planetary naming is that the closest planet to the star (if multiple planets are found at the same time) is named "star"b, then "star"c and so on. As correctly pointed out by Zephyr, if the discoveries are more haphazard, the order of discovery takes precedence over distance from the star).
So, there is no Trappist-1a. Or you can think of it being the star itself if you like, although that becomes confusing if the star is itself part of a multiple star system, where it might be known as "star"A or "star"B etc. (note, capital letters), depending on where it is in the hierarchy.
Interesting. Any idea where that convention started? I'd guess somebody somewhere called the star "star"a, and it became a convention, but I know nothing.