How to remove full-disk(hard drive) encryption from Ubuntu Mate?
Thank You
I don’t think you can. You will need to back up your data to another disk or partition and then reinstall.
The other problem is in the BIOS in boot selection it doesnt show me the option to boot from CD/DVD.
Did it used to or it never has?
Look how the thing was done, I install the Ubuntu Mate in the tradioniotal way F9–> Boot Selection --> Boot from CD/DVD i put the full encryption of the hard disk so far so good the software was installed successfully. Because i have the devil inside me i think what if i want to replace this distro with another distro of linux, i start the pc Boot Selection again the only option was Boot from Hard Drive.
Installing an OS shouldn’t change the BIOS boot options. Did something else change, such as BIOS settings? (E.g. “boot to external device” or “secure boot” settings.) Does it only show the boot to DVD option when a DVD is inserted?
With DVD or not doesnt show it and also in the OS I insert the DVD you hear the sound of reading but it doesnt pop up anything.
When you boot, there is a button on your PC to select the boot order, it might be Esc or F12 or maybe something else, the next time you boot and you see the very first window, press the pause button and take a look and try changing the boot order!. 
Hmm, it could be that the DVD drive had failed, but if it was working recently that’s sort of odd. It’s hard to say much more without seeing it. I take a look in the BIOS settings andsee if there’s anything about boot options or boot order as Woldman suggested.
It was hardware the problem thanks for the help. In my original question is there any manual to remove full encryption?
As far as I know, you will need to back up your data to another disk or partition and then reinstall without encryption.