Let's get some facts ...
are you trying to install UM (ubuntu-mate) on a laptop that has Ubuntu already installed?
do you want a dual boot system or completely wipe the disk (sda) and install UM?
do you have the specs of the laptop (model, disk, ram, graphics card)?
When you boot from the usb stick, it offers 1. Try Ubuntu or 2. Install Ubuntu. Try will let you test that UM loads and works well (graphics, wifi, etc.). Then, you can click on Install UM which will start the Installer. This is where you decide to wipe your disk and install UM or install alongside another OS.
Let us know, there are guides in this forum for installation.