I need to run a report by category? for example I am trying to find out how much I have spent on Sub Contractors

I'll provide the two options to view the total amounts you've spent to your sub-contractors, @laura90. 

If you're recording payments manually in QuickBooks Online (QBO), let's run the Expenses by Vendor Summary report. It lets you see your total expenses for each vendor. Please take note that we categorize sub-contractors as vendors in the program. With this, there isn't a need to run a report by category. I'll guide you how. 

1. Go to Reports from the left menu. 
2. Enter Expenses by Vendor Summary in the Search box. 
3. Set the Report period. 
4. Choose the appropriate Accounting method. 
5. Click Run Report.

The screenshot below shows you the last three steps. 

I recommend clicking Customize at the upper right of the report. From there, you'll be able to apply multiple filters to focus on the details you need the most. To learn more about this process, see Step 2 through this article: Customizing Reports. 

If you're using Online Bill Pay when paying your sub-contractors, let's go to the Sent Payments tab. You'll find a list of all sent payments through this service. Here's how: 

1. Go to the + New button from the left menu. 
2. Select Pay Bills Online under Vendors. 
3. Choose Sent Payments.
4. Review the total payments of each sub-contractor. 

Also, you can use the Payment Tracker feature from the Transactions tab. It lets you track the status of your sub-contractor payments. 

I'm just a comment away if you need further assistance. Take care always, @laura90.