[SAMD51] Postmortem Debugging not implemented (#21211)

Marlin/src/HAL/SAMD51/inc/SanityCheck.h

@@ -50,3 +50,7 @@
 #if ENABLED(FAST_PWM_FAN) || SPINDLE_LASER_FREQUENCY
   #error "Features requiring Hardware PWM (FAST_PWM_FAN, SPINDLE_LASER_FREQUENCY) are not yet supported on SAMD51."
 #endif
+
+#if ENABLED(POSTMORTEM_DEBUGGING)
+  #error "POSTMORTEM_DEBUGGING is not yet supported on AGCM4."
+#endif