I can't boot to Sierra 10.12.6 without the USB flash

[P1LGRIM]

Still no

Then something is wrong with your installation.
It is probably quicker to start from scratch and reinstall than it is to troubleshoot your existing installation.
Your choice.

 

[kartagis]

1. I boot off USB flash.
2. I complete the installation
3. I install Clover to the main SSD.

Is that all?

 

[P1LGRIM]

1. I boot off USB flash.
2. I complete the installation
3. I install Clover to the main SSD.

Is that all?

Just follow the Installation Guide.