They already had to cut out a big chunk from the middle of The Martian (which then broke the I've got no radio, so no one can give me permission to board, so I'm now the first space pirate joke, which they decided to keep anyway), and Project Hail Mary is a much longer book - it's about as thick as Dune, which became two movies and still cut loads. Something major will have to change if they're going to give it an acceptable runtime.