There are several possible reasons for this error.
A large time step has caused an error in the matrix used to solve this analysis
Part modeling might be flawed
Low Heat Transfer Co-efficient (HTC)
To resolve a time step error:
In the Cool Solver Parameters dialog, increase the Number of time steps for 3D cooling flux calculation to 100 and re-run the analysis. This will increase the analysis time but the analysis could converge to a solution.
To resolve a modeling error:
Run the Mesh Analysis Wizard and resolve any problems
Re-mesh the part
Re-import the model
If there are part inserts, ensure matching surfaces are meshed with the same edge length, and the surfaces are not intersecting.
To resolve a HTC error:
Try using a HTC value of 30,000 on all matching interfaces between the part and the insert. If the solution converges, redefine the HTC values.