$AS$5 is checkbook cleared balance. $AG$7 is checkbook non-cleared balance. If $AG$7 is blank, #VALUE! is shown.
What about =SUM($AS$5;$AG$7) ? Not “plus”, but “semicolon”?
If so then AG7 is not a blank cell but contains some text, for example a space (blank) character, i.e. ISBLANK(AG7) returns FALSE. Or it contains a formula returning an empty string. The behaviour also depends on the detailed calculation settings how strings are to be treated as operands.
Apart from that, =SUM($AS$5+$AG$7) is an unnecessary SUM() call, =$AS$5+$AG$7 would be the same.