Paul,
A jacobian element is a highly distorted solid element (like a brick or tetrahedral) that would cause numerical problems during the solution. If the problem is in a part from a CAD solid model, then you can re-make the solid mesh in part with the problem. Do a Search in the in-product documentation ("Help > Search") for Jacobian. One of the hits will be the page "Nonpositive Jacobian Errors or Distorted Elements" which describes the procedure.
I presume you found the log or summary file that gives the warning message. I hope that the transient heat transfer processor indicates which part and/or element contains the jacobian problem.
Sincerely,
John Holtz
Product Design Engineer, Algor Simulation
Autodesk, Inc.
John Holtz, P.E. Global Product Support
Autodesk, Inc. If not provided already, be sure to indicate the version of Inventor Nastran you are using!"The knowledge you seek is at knowledge.autodesk.com" - Confucius 😉