He was (evidently) told that he couldn't include Luke. He was eventually allowed to include a Han Solo story, presumably after it became clear that the Han Solo film would be a prequel
Q. Were there any restrictions on you, like, you can’t have Luke Skywalker, you can have Han Solo — which is one of the interstitial
stories in Aftermath.
Chuck Wendig: There is, yeah, there’s a Han [story.] There were definitely some restrictions. There were things that I had written
that they were like, “we need to pull back on this for various
reasons.” The [Lucasfilm] story group has got this massive, braided
universe — there’s a mobile game called Uprising; obviously you have
[the EA-DICE video game] Battlefront; you have the film, clearly; and
then the novels and the comics. So it’s always trying to balance that
stuff. So it’s like, “Well, you can talk about that character here
because this character’s there, but then we want this character to be
exclusive to this other thing, so you can’t mention them…” But for the
most part, I had a few clear restrictions up front, and then I was
allowed to run with it.
Star Wars: Aftermath: Finding Han Solo
He speaks to the fan-pressure to include Han and Luke in his books (and the more irresistible pressure from the Story Group not to do so) in the interview below.
The thing with Aftermath, [is that] it was tricky was, and this is where the
pressure really came in, is it came with this rolling thunder storm
of expectation right because it people, everybody wanted something
different from that book. They wanted Han and Luke or they wanted a
continuation into the The Force Awakens or they wanted secrets into
The Force Awakens and the story I was telling was not really able
to do those things...