Hello when I press shift + r in the sculpt mode, a weird grid appears: 
In Edit Mode/object mode it works with no problems.
The current Blender version binds Shift-R to "edit voxel size". When moving the mouse you can change the density of the grid. When you later do a remesh that resolution corresponds to the size of the resulting faces.
Shift-R is useful in edit or object mode, where you work with specific commands. In sculpt mode I don't really know what you would want to repeat.
I agree that SHIFT+R is really useful in Sculpt Mode. Like ddadim said, the shortcut is overridden by "Edit Voxel Size.So here is what you can do: in your Preferences, Keymap > 3D View > Sculpt > Sculpt (Global) change the Shortcut to something other than SHIFT+R (I am using ALT+R), that way you get the repeat last operation functionalityt of SHIFT+R back in Sculpt Mode:
