"A circle with a line or slash through it means that the selected startup disk contains a Mac operating system, but it's not a macOS that your Mac can use. You should reinstall macOS on that disk." - Apple Support
When you restart you Mac, press and hold the Option key until the startup disk choices appear. If the usb installer doesn't appear then you'll have to redo the installer using the link in my previous post.
Please also look at a couple other options you have when restarting the Mac:
Command (⌘)-R to reinstall the latest macOS that was installed on your Mac (recommended).
OR
Shift-Option-⌘-R to reinstall the macOS that came with your Mac, or the closest version still available.
See more about these options here:
How to reinstall macOS from macOS Recovery - Apple Support