When "Fast IRC Regulator" check box is checked (in below figure the checkbox is not checked) bit "FIRCREGOFF" from register "FIRCCSR" in System Clock Generator (SCG) module is SET (=1). That means the regulator is OFF.
This seems to be in the opposition with the description of the tool-tip (below image).
To have the FIRCREGOFF bit to 0 (FIRC Regulator enabled) I have to un-check the box (like is below).
I use S32 Design Studio for ARM, Version 2018.R1, Build id: 180815
I hope this helps.