To get to this option you go to Settings > Invoices, Estimates & Receipts
Once you're there, you scroll all the way to the bottom of the page and look for the "Require Card on file to accept Estimates on Client Portal" button
From here you can choose from the following options:
For no customer
For all customers
Using the setting determined in customer profile
The first two options explain themselves, but for the last option, you can find their customer profile setting in... you guessed it... their customer profile!
Once you're in your customer's profile, you can go to the Client Portal module
After you expand the box if it isn't already, you can check/uncheck the Credit Card Required box to your liking
This setting only matters if you clicked the Using the setting determined in customer profile option in your global settings