To resolve this issue , I'll suggest the following:

Create a destination variable e.g Dest_Var

Set its EvaluateAsExpression to "True", and copy and paste the expression you created above into the variable's Expression property

For the File System Task:

Set its IsDestinationPathVariable property to "True"

Set its DestinationVariable propety to the destination variable you created

(usually in the format-scope::Dest_Var)

Set its IsSourcePathVariable property to "True"

Set its SourceVariable property to the source variable you created

(usually in the format-scope::Src_File)

And finally, delete

DelayValidation = True, and the

Destination property you've set in the Expressions tab.

I hope this helps.

Cheers!