how can I fix the "digital signature for this file couldn't be verified" Windows boot manager error?

There have been a series of issues leading up to this, but only within the last few days has it gotten to the point where my Acer One 10 convertable laptop is no-longer usable. After difficulties installing Ubuntu on my device, I went to Acer customer support for help, and after following their instructions, I still couldn't boot to Ubuntu, and an error kept comming up in windows: "Re-install windows now, this build of Windows has expired. This PC will restart every few hours and eventually fail to start."

I try to re-install windows, but whenever I do, the computer restarts, and on the Acer logo page, it says: "Undoing changes". Then I am back with my old version of windows with the same errors. And just three days ago, my computer boots into a black Windows Boot manager screen, saying that the digital signature for the Winload.exe couldn't be verified. Is there any way to fix this? And if so, is there a way I can still get Ubuntu on it?