Are you sure that in that same workbook you have a sheet named Travel Task
2?
If you are still struggling, insert the formula again by typing the = sign
then going to your Travel Task 2 sheet and clicking in cell H20, then accept
the formula.
How about the sheet name? If it's even slightly misspelled, you'll get this error. Try
rebuilding the formula, either as a formula (type =, go click H20 in the other sheet, and
press Enter), or as a copy from H20 and a paste-link into your original cell.