Calculated Date Difference Question

Before submitting this question, I’ve tried researching this in the forums and have attempted multiple variations of suggested formulas with no success. So apologies in advance for asking what seems like a stupid question but I’m no guru with formulas as you can see.

I’m working with SP Online and trying to create a simple calculation to show the span of business days between submission and approvals from our customer. Currently have the below calculation but this gives total days including weekends.

=[Date sent to Customer]-[Customer Approval Date]

I’ve tried to use this next formula to take business days into account but it doesn’t like it. What am I doing wrong?

=NETWORKDAYS[Date sent to Customer]-[Customer Approval Date]

Any help would be GREATLY appreciated!