Within xASL_module_Population.m , the variable x.S.Atlases is:

{'TotalGM'                   }
{'DeepWM'                    }
{'MNI_Structural'            }
{'Hammers'                   }
{'HOcort_CONN'               }
{'HOsub_CONN'                }
{'Mindboggle_OASIS_DKT31_CMA'}

Meanwhile the atlas filename is MNI_structural.nii.gz

Not sure which one you want to settle on, but the difference is naming is preventing that atlas from being included in the analysis while running the Population module. For what it's worth, the published version of the GUI currently outputs MNI_structural when creating the DataPar file. If you settle on the titlecase convention, please let me know as well.