choice of heat sink (thermal resistance)
The result will be positive for practical passive setups.
A negative thermal resistance for your heatsink to ambient calculation means you are trying to power the device at more than its internal thermal resistance can handle. That's why it wants you to have a negative thermal resistance to ambient.
The solutions may include

reduce the power that you're dissipating in the device
see if the device has an alternative package with a lower RthJC
use a lower ambient temperature
if necessary mount the device on a Peltier to reduce its case temperature