subtotals by account in the category report

Based on the question category, I'm going to assume you are using Quicken for Mac current subscription version,

Click the Reports menu, then New Report, then Summary. For Rows, choose Categories. For columns, choose Account. Click Create Report.

This will give you a matrix with categories down the left side and accounts across the top. You can further customize to limit to just certain accounts and/or categories. Click on any individual entry to see a popup of the transactions that make up that subtotal.