I acquired this bike from a neighbor. I plan on turning it into a gravel bike with a one by drivetrain. Can anyone tell me if it can fit an 11 speed cassette in the rear?

I would try riding it first. It’s a good idea to make sure it’s the right size. Throw some grease on the rusty chain to free it up and ride it around for a few miles before dumping money into it. There are a lot of good bikes out there for less than it would cost to fix this one and turn it into a gravel bike.