CodeGrade doesn't understand '-std=c23'

makefile

@@ -1,4 +1,4 @@
-CFLAGS = -std=c23 -Wextra -pedantic -Wall -O0 -g3 -fsanitize=address
+CFLAGS = -std=c11 -Wextra -pedantic -Wall -O0 -g3 -fsanitize=address
 LDFLAGS = -lncurses -fsanitize=address
 
 spel: box.o main.o ui.o util.o chart.o

ui.c

@@ -30,7 +30,7 @@ ui_init(void)
     refresh();
 }
 
-void ui_end()
+void ui_end(void)
 {
     endwin();
 }