Try entering the full command (TUI might be auto-completing your arguments).
/mesh/adapt/cell-registers/add region_0 type hexahedron inside yes max-point 0.3 0.5 min-point 0 0
"inside" might need to be replaced as "inside?", I invite you to try this.
One way to test what the full command should be is to write it in the TUI as multiple lines. The TUI will then suggest to you the full input.
For example,
/mesh/adapt/cell-registers/add
region_0
type
hexahedron
inside?
yes
max-point
0.3
0.5
min-point
0
0
You are navigating away from the AIS Discovery experience