Why 10.0.64.0/24 overlaps with 10.0.66.0/18?

They overlap in that 10.0.64.1 is in both 10.0.64.0/24 and 10.0.66.0/18, for example.

It's not the /24 range which is causing you a problem - it's the /18 range, which will include everything from 10.0.64.0 to 10.0.127.255.

The "66" in 10.0.66.0/18 is pretty weird, to be honest... if only the top 18 bits are relevant, why is the 23rd bit being set?