Nothing came out from Functions maybe here?

Sheet1 - logged ticket books to sales persons

name beginno endno
a 51 100
b 101 150


I do below checks by vba if user makes a wrong entry say


d 75 - msgbox " Number already in use "
e 151 141 msgbox " Wrong entry"


But couldn't do below check


f 1 200 msgbox " Numbers already in
use"


Any help ? Thank you