Can share my Mac (2017) screen to my Macbook M1 Pro (2020) over thunderbolt, but not the other way around.

Just like the title says, I'm trying to use my old mac as a second monitor for my macbook, but it won't show up on the screen mirroring options. Again, my macbook shows up for the mac, but the mac doesn't show up for the macbook. any ides on how to fix this?