How can i solve the problem of natural convection? and coupling of physicis?

Hi all.
I want to analyze a heat sink in natural convection.
I gave Temperature under the heat sink(80 degC), outside open boundary and volume force.

when I pushed study, I have a problem.
That is follewed,

Undefined variable.
- Variable: comp1.nitf1.T
- Geometry: geom1
- Domain: 1
(I attatched the jpg file.)

maybe, I think non-isothermal flow(nitf) have several problems, but I can't find it.
so how can I solve this?
Thank you. have a nice day!