Q: booting from boot camp assistant created USB drive

What year/model is your Mac? Please do not post your Mac's serial number.

Can you check Bootcamp.xml on the USB and look in the file for lines which are similar to the following?

  <ProductManufacturer>Apple Inc.</ProductManufacturer>

  <ProductVersion>5.1.5621</ProductVersion>

<ProductCode>{FA2B2C2A-EA41-495A-9308-60726125D562}</ProductCode>

Please post the line from the file.

This is an invalid version for installing Windows 7.

Please download the correct version for Windows 7 from Install Windows 7 and earlier on your Mac using Boot Camp - Apple Support based on your Mac year/model to a USB. Create a new folder on your Mac and copy the contents of the USB to this new folder. Run BCA and create your USB (but do not partition your disk - uncheck the box for partitioning/install). After BCA builds the installer, replace the contents of $WinPEDriver$ on the USB with the new folder containing  the correct $WinPEDriver$ directory. Repeat the same for the Bootcamp folder and AutoUnattend.xml.

Once you have done these steps, choose the third option (Install...) on BC Assistant and partition and install. If you run into any issues, please post back.

In Finder, can you open the USB and post a listing of files? It should similar to this. Expand $WinPEDriver$ and Bootcamp folders.

Please also run the following two procedures before your next installation attempt.

Resetting the System Management Controller (SMC) on your Mac - Apple Support

How to Reset NVRAM on your Mac - Apple Support