Marcia

You cannot use ISNA in the above case.
Select D4 and (in the Formula field) highlight exactly the following part of the formula:
=INDEX($B$2:$B$7,SMALL(IF($A$2:$A$7=TODAY( ),ROW($B$2:$B$7)-ROW($B$2)+1),ROWS($B$2:$B4)))


press Ctrl+C to take a copy and paste it in an empty cell (say I4). Now put an equal sign in front of this and hold down Ctrl and Shift before pressing Enter to make it an array formula. With I4 selected press Evaluate Formula and press the Evaluate button 8 times after which you will see the following:
INDEX($B$2:$B$7, SMALL(IF(FALSE, FALSE, FALSE, FALSE, 5, 6), 3))
But the SMALL function can't return the third smallest value from an array with only 2 values (different from FALSE). That is why you get the #NUM! error.
And ISNA(#NUM!)=FALSE, so finally you have =IF(ISNA(#NUM!, "", #NUM!) which returns #NUM!

Thank you Xor for the precise instructions on how I should evaluate a formula. I didn't know about this process. I could now analyze how a formula with several nested functions work.