What does this error mean when I run the FDTD simulator?

When you right-click on the error, you will see that you do not have enough HPC. 

If you would like to use all your cores, you will need to add HPC licenses to Lumerical Enterprise.

https://optics.ansys.com/hc/en-us/articles/360058577794-Lumerical-solve-accelerator-and-Ansys-HPC-license-consumption#toc_4

Please visit this HPC calculator to know how many HPC you need.

https://webapps.ansys.com/era/ssosupport/parametric