I don’t mean to sound more knowledgeable  than you.

But It’s difficult to assist from here, here is is what I would do. 

1- check boot sequence against, make sure it sees the USB stick. If this is good. 
2- burn the OS iso on stick again. 

The OS should be installed on the internal drive of course, but of course carful you keep your backup away from the computer so that you don’t format it by mistake. 

 You were right. It was the boot sequence.