First of all, make sure that the U disk has been started by U, and the U disk is ready, which means that the U disk must have the startup function. If not, please go to U Boot official website to download the U Boot U disk startup disk making tool, make the U disk into a startup U disk, and then try again.
2. If it is confirmed that the USB flash drive has been booted, the first boot option of BIOS is set to USB flash drive, which means it can't be detected. It is estimated that the computer started too fast. It is recommended to restart the computer several times and try again. Third, if you have tried all the above methods, it is likely that your computer has disabled the USB boot device. Solution: Just boot into the BIOS setup interface, find the English word about USB, and then change "Disabled" to "Enbaled" in this option.