Stuck with Multiple If Conditions

This should do the trick, however you need to reconsider your logic for whenever [Days] = your criteria.  All of your statements deal with Above and Below, but never on the same day.  For the below i've had to assume what you wanted.

Status = IF(
	ISBLANK([LastDate]),
	"OverDue",
	IF(
		[Service] = "JAQ",
		IF(
			[Exception] = "Yes",
			IF(
				[Days] < 8,
				"OnTime(E)",
				"OverDue(E)"
			),
			IF(
				[Days] < 15,
				"OnTime",
				"OverDue"
			)
		),
		IF(
			[Days] < 20,
			"OnTime",
			"OverDue"
		)
	)
)	

Could you try something like....

Status = SWITCH (TRUE() ,
[LastDate] = blank() , "OverDue"
[Service] = "JAQ" && [Exception] = "Yes" && [Days]  < 8 , "OnTime(E)"
[Service] = "JAQ" && [Exception] = "Yes" && [Days]  > 8 , "OverDue(E)"
[Service] = "JAQ" && [Exception] = "NO" && [Days]  < 15 , "OnTime" 
[Service] = "JAQ" && [Exception] = "NO" && [Days]  > 15 , "OverDue"
[Service] <> "JAQ" && [Days]  < 20 , "OnTime" 
[Service] <> "JAQ" && [Days]  > 20 , "OverDue"
-----ELSE------
"Other")