That error means "Bad index" (
see this link[
^])
If you don't know what the name of the sheet is, just that it will always be the first sheet you could do:
xlBook.Sheets(1)
another option is if the user inputs the name of the sheet you could iterate around each sheet comparing a trimmed version of the name. The .Trim() function can be used to remove all leading and trailing whitespace characters from a string. This wouldn't be very efficient however as it does need to go around each sheet.
A third option, would be to get a list of sheet names and ask the user which one they wish to access.