I'd say update your graphics drivers, but since you're on Mac that's a bit tricky. Firstly check if it's an issue with your footage, try the same operations on another clip. If it crashes with any clipc, then check if you have any mac updates waiting. If that doesn't work, try re-installing AE.
Color Range keyer is a very old plugin and it's not really that good. If you could post a screenshot (embed please using the toolbar) we could give you some suggestions for a better way to pull the matte. You also might want to roll back one version of AE unless you are running Catalina. If you are on Catalina I strongly suspect that the OS is not compatible with the combination of effects you are using.
Thanks Rick. Maybe it's the age of Color Range. Question - if color is the ideal keying method - which plugin would you use if not Color Range? ( You are correct, I am indeed on current ver. Catalina BTW )
The souce material is really difficult no matter how you slice it. The keyed color is beige, with shadows - ugh. The subject is a person with skin color that is pretty close. Contrast is similar. You get the issue. Even so, I was getting *good* keying results from tweaking the heck out of Color Range - there was a subtle hue difference and it seemed to be enough to pull a key - but with the crashing I finally gave up. It's disappointing because it seemed I could have done it otherwise.
I have chosen all new backgrounds that are a similar in color/texture to the beige wall and have masked the subject instead. It's not as good, but if you're not looking closely you don't notice the masking.