Syntax error when looking at difference between project completion date and today

Hi! 

I’ve searched the forums endlessly to try and find a solution, we are trying to get Lists to calculate the number of days left between today’s date and when the project is due to complete. I have tried the below formula, replacing with relevant columns, but get syntax error as per image 

"=(Number([$DueDate])-Number(@now))/86400000"

Does any one know how to fix?