Throw an error in M420 if can't enable

Marlin/Marlin_main.cpp

@@ -2243,7 +2243,13 @@ static void clean_up_after_endstop_or_probe_move() {
 
     #elif HAS_ABL
 
-      if (enable != planner.abl_enabled) {
+      #if ENABLED(AUTO_BED_LEVELING_BILINEAR)
+        const bool can_change = (!enable || (bilinear_grid_spacing[0] && bilinear_grid_spacing[1]));
+      #else
+        constexpr bool can_change = true;
+      #endif
+
+      if (can_change && enable != planner.abl_enabled) {
         planner.abl_enabled = enable;
         if (!enable)
           set_current_from_steppers_for_axis(
@@ -7007,10 +7013,27 @@ void quickstop_stepper() {
    *       Z[height] Sets the Z fade height (0 or none to disable)
    */
   inline void gcode_M420() {
-    if (code_seen('S')) set_bed_leveling_enabled(code_value_bool());
+    bool to_enable = false;
+
+    if (code_seen('S')) {
+      to_enable = code_value_bool();
+      set_bed_leveling_enabled(to_enable);
+    }
+
     #if ENABLED(ENABLE_LEVELING_FADE_HEIGHT)
       if (code_seen('Z')) set_z_fade_height(code_value_linear_units());
     #endif
+
+    if (to_enable && !(
+      #if ENABLED(MESH_BED_LEVELING)
+        mbl.active()
+      #else
+        planner.abl_enabled
+      #endif
+    ) ) {
+      SERIAL_ERROR_START;
+      SERIAL_ERRORLNPGM(MSG_ERR_M420_FAILED);
+    }
   }
 #endif
 

Marlin/language.h

@@ -157,6 +157,7 @@
 #define MSG_ERR_MESH_XY                     "Mesh XY or IJ cannot be resolved"
 #define MSG_ERR_ARC_ARGS                    "G2/G3 bad parameters"
 #define MSG_ERR_PROTECTED_PIN               "Protected Pin"
+#define MSG_ERR_M420_FAILED                 "Failed to enable Bed Leveling"
 #define MSG_ERR_M428_TOO_FAR                "Too far from reference point"
 #define MSG_ERR_M303_DISABLED               "PIDTEMP disabled"
 #define MSG_M119_REPORT                     "Reporting endstop status"