Thanks @jjm for your help. In fact I could not make too much change as there was already data on my /home partition.
I removed anyway /boot/efi and finally figured the solution (@jclvanier also give me some hints), each time the usb started in EFI mode. But the partitions are not efi. So I could start USB stick in legacy mode, reinstall. I had no warning about no «/boot/efi» partition, and all finally went well, I’m writing from Lx 3 