I just transfered my company file to the new QB 2020 pro. When I tried to read in my transactions, QB would not let me select my existing checking account. What do I do?

Welcome to the Community, @briancarolina-ms.

Since you've switched to the new version of QuickBooks Desktop, you'll have to update your company file too to make it work. Here's how: 

1. Close your company files and QuickBooks, then open the Windows Start menu.
2. Search for QuickBooks Desktop, then right-click the icon and select Run as administrator.
3. While on the No Company Open screen, click the Help menu and select Update QuickBooks Desktop.
4. Go to the Options tab, then select Mark All and click Save.
5. Click the Update Now tab, then select the Reset Update checkbox.
6. Select Get Updates. 
7. Close and reopen QuickBooks Desktop when the update is done, then select Yes to install the updates.

Once everything's done, restart your computer for the changes to take effect. For additional insights about upgrading to the newest version, read through this article: Upgrade your company files after switching to a new version of QuickBooks Desktop.

Should you have payroll subscription as well, ensure to download the latest payroll tax table so you can process payroll in QuickBooks Desktop.

Let me know if you need help with anything else. We're always here to assist.