This is one of those cases that the program lacks some rounding skills. Your alignment stops at 9+071.9583, but since the stations are only displayed to 2 decimal places 9+071.96 is displayed as the default. By hitting Enter to accept the default, it uses the .96 literally and sees it is past the .9583 and disallows the entry. If you select the actual end of the alignment using the endpoint Osnap (you must select it twice, not sure why) it will work as desired.