You can create a variable to store this like → daysBetween($Instruments/ContractExpiry,$Instruments/Date)
in your case its comparing them whether they are equal or not
$Instruments/Esculation=daysBetween($Instruments/ContractExpiry,$Instruments/Date)
You likely have an activity ‘Create variable’ expecting an integer and in the expression of that activity you have this:
$Instruments/Esculation=daysBetween($Instruments/ContractExpiry,$Instruments/Date)
which results in a boolean. Which is exactly what the error says: I expect a integer, but your expression returns a boolean.
Replace = with < will remove the error.