How do I create an expense report by category (not supplier)?

Hi there, 

Congrats on your first post in the Community. I'll be happy to help and share more info about running reports in QuickBooks. 

Based on your description, I recommend running a Transaction Detail by Account report. We can customize a Transaction Detail by Account report to show your expense transactions. Let me guide you how to do that:

1.     Go to the Reports tab.
2.     On the Find report by name field, look for Transaction Detail by Account.
3.     On the report, click the Customize button.
4.     Click on Filter to expand the options.
5.     On the Transaction Type drop-down, select Expense.
6.     In the Account drop-down, mark the accounts you'd like to view in the report.
7.     Click Run report.
8.     Once done, click the Report period drop-down and select This Calendar Year or This Fiscal Year.
9.     Click Run report.

Next, you can click the Save customization button to retain the changes applied to the report. This way, you no longer need to repeat the customizations the next time you get the report.

To access the customized report, follow the steps below:

1.     Go to the Reports menu.
2.     Click the Custom Reports tab.
3.     Select the modified report. 

Try out these steps and let me know how you make out. I'll be on standby. :)