It looks okay. Only thing you have to be careful about is while using this variable, use with '$' symbol, i.e. call this variable like $(vExecutionSummaryIntakeOrder)
And yes, there is a catch, why are you using '{}' while comparing? Try without it like:
LET vExecutionSummaryIntakeOrder = 'sum(if([NB_Execution_Summaries_Table.sort order]=10,[NB_Execution_Summaries_Table.Order intake today]))';