Dual booting Lenovo Thinkpad X1 Windows 10 with Ubuntu Mate?

If 'Install alongside Windows' option is missing, it's better to try out 'Something else'. Here you have to manually format the new partition you have created. You should atleast have the following two:

1) Swap: Select the partition and click on '+' symbol to add swap. Allocate memory equal to your RAM size and select 'Swap' in use as option.

2) Root: Again click on '+' and use following configuration:

Size = Your remaining partition space

Type for the new partition = Primary

Location for the new partition = Beginning

Use as = EXT4 journaling file system

Mount point = /

Now continue with 'Install now'.

Refer these articles for more information: https://www.tecmint.com/install-ubuntu-16-04-alongside-with-windows-10-or-8-in-dual-boot/

https://itsfoss.com/install-ubuntu-dual-boot-mode-windows/