Welcome to the forum,
Try:
Excel Formula:=ROUNDUP(SUM('A1 2-Bed Hot Tub'!F84*110%),0)
The zero at the end denotes how many digits to round to, zero will return a whole number.
=SUM('A1 2-Bed Hot Tub'!F84*110%)
=SUM('A1 2-Bed Hot Tub'!F84*110%)
=ROUNDUP('A1 2-Bed Hot Tub'!F84*110%,0)
=ISNUMBER('A1 2-Bed Hot Tub'!F84)
That is NOT an error.=ISNUMBER('A1 2-Bed Hot Tub'!F84) and get the FALSE Error.