The Application.InputBox function is very useful to get a range from
the user. Unfortunately, this function exposes a bug in Excel (all current
versions!). If the sheet on which a (range of) cell(s) is selected contains
conditional formatting using the : “Formula Is” option, the function may fail,
returning an empty range.
The only reliable workaround is to build a userform to request the range from
the user, which I have included as a download here.