How do I write this into the formula to make the #VALUE! blank?
=IF(ISERROR(VLOOKUP($B10,Claims!$A$1:$G$18591,2,0)),"",VLOOKUP($B10,Claims!$A$1:$G$18591,2,0))/1000
The issue is when the VLOOKUP returns an error you are trying to divide a null string by 1000.
So add the division into the check:
=IF(ISERROR(VLOOKUP($B10,Claims!$A$1:$G$18591,2,0)/1000),"",VLOOKUP($B10,Claims!$A$1:$G$18591,2,0)/1000)
But, there is no need for the use of ISERROR, wrap in IFERROR:
=IFERROR(VLOOKUP($B10,Claims!$A$1:$G$18591,2,0)/1000,"")
It avoids the need of duplicate enteries.