How do I add Sub Categories for purchases?

Not sure what you are asking

QBO uses the word category twice, to mean different things

in the product and services list category means what you think, a way to organize the list

in the chart of accounts, category means account, you create an account of a type and name it.  Intuit tried to, I guess, make it easier with the detail type.  Just use an expense account detail type other business expenses if you can not find a detail type that says what you want

In the US version detail type utilities is all the way at the bottom, scroll down you may have missed it

Hey Kings Oak Accountants,

Welcome to the UK Community Page for QuickBooks.

To mimic Rustler's response, i'm not 100% sure what outcome you're trying to achieve here, what function would you want the sub-categories to perform on your books? What would you ideal end result look like?

To add to Rustler's response - you can create "Sub Accounts" - and set an Expense account up as a sub-account to another expense account, giving it a parent/child set on your Chart of Accounts.