Messed up the boot... how do I create partitions now? [duplicate]

I'm currently using Ubuntu 20.4.1 and I need to create partitions in the disk. I didn't create any during the installation. I used Gparted but it doesn't seem to be working as the disk needs to be unmounted before resizing and I can't unmount the disk as there is only one partition (which is being used now). So how do I create partitions now??

This is the image when I tried to unmount:

And this is when I try to use Gparted. The slider doesn't move and if I put numbers in the 'free space following(mib)' option and press enter, it just resets to zero:

As you indicate in your question, your one partition that was automatically created during installation is in use. You cannot change a partition that is in use.

To change the partitioning on this drive, you will need to work from a live session, i.e., a session started from an installation DVD or USB. More advanced users can do this at the terminal from a recovery prompt.