I just done this myself a couple of days ago and here's who i did it... (from memory so might not get the exact names correct)
Firstly you need to disable bitlocker. Boot into windows 10. go to System. There should be an option for device encryption where you can turn this off. Depending on your system and disk space etc it will take quite a while. Mine took approx 30 mins.
Next go to Disk Management, select your C drive, right click and select Shrink. Your system will then calculate how much you can shrink this partition by. I ran into an issue at this stage and it was because i had System Restore on and it must have been stored on a later part of my drive. If this is the case you can disable system restore temporarily and delate any existing restore points.
once you've freed up 'empty space' on your disk, you can proceed to boot with your flash drive again and install Ubuntu as before. When it asks you if you want to install alongside Windows, Select 'Something Else' where you will then be taken to a screen where you will see your partitions. select the 'free space' that you just freed up, format as ext4 and mount to '/'
on the dropdown underneath this select that you want to install grub to the same partition as your Windows Boot Manager and proceed with the installation. This should hopefully work.
Good Luck!
