Installing nvidia drivers and using nvidia graphic card

[MartyBuntu]

It really looks like Secure Boot is still enabled.

It would appear so.

[marcb2]

Thanks for your answers !
I just re-checked : secure boot is disabled, as well as switchable graphics...
I am on the verge of re-trying shell installation using nvidia's system...

OK, so I finally solved the issue : nvidia drivers were blacklisted in /lib/modeprob.d/... so had to get rid of that. Realized that when trying to install from nvidia's shell installer thanks to its error message, so I let it run like that rather than trying to re-install everything using the ubuntu interface... I write this here in case it turns out to be useful to someone. And thanks again for all the help !