It's great to have you reach out in the Community. Sometimes it's necessary to delete the expense categories so you can focus on the rest of your work with peace of mind. QuickBooks Online makes it simple to manage your income and expenses using the powerful features in the program.
Since the expense category is attached to the Chart of Accounts (COA), you'll have to delete the account associated with the expense category in order to delete. It's important to note that an cannot delete an account. however, you can make it inactive. Doing this stops the expense category from your list when creating a transaction.
Here's how to make an account inactive:
Navigate to the Accounting menu.
In the Chartof Accounts tab, find the account you want to make inactive. You can utilize the search box to easily find the account.
Once you find the account, click the drop-down arrow under the Action column.
Then, select Make inactive.
Give this a try and let me know how you make out. I'll be one message away in case you need anything else.