Bootable USB can't enable from boot

Hello

I'm Deep

I want run Ubuntu OS from my pen drive, I successfully installed it in USB 3.0 and also successfully run it through USB but I faced problem when I don't want use Ubuntu therefore I don't use Ubuntu boot pen drive but that time I face grub error and I tried to solve this using bootsect / fixmbr etc. Now the problem is solved but I can't run Ubuntu from USB flash drive bcz I can't boot it from boot menu, there I can't see USB drive selection menu.

Thanks.