[SOLVED] Error installing along side Windows 10

[stevef]

I tried mounting the Windows partition using the Disks app and it mounted this time.

It is odd that the Windows partition mounts ok now. Something must have changed, but the initial failure to mount was a distraction.

I found a choice between Legacy and UEFI.  When I select UEFI, it shows ubuntu as the only OS to boot to.  If I choose to add another choice, it says that it can't find a valid file system.

Sorry, I was trying to get information about your set up, not asking you to change it.
Can you confirm what choice was active when it was a working single boot W10 system (before the LL installation was done).  If you made any changes to the BIOS/UEFI during the installation of LL, let us know.

/dev/sda1:Windows 10:Windows:chain

I think this confirms W10 is still there, but one of the problems we may have is the Linux boot loader (GRUB) is not being informed about the existing W10 installation.  This may be due to a fairly recent policy change in the set up which stops os-prober running by default.  This gets more complicated if the original W10 system is Legacy mode (which was the reason for asking the question about the Legacy/UEFI set up).

Originally, your question was around getting Windows 10 back.
Is this still your priority ?
Do you have a system backup taken before the LL installation ?