Sounds like you have a small drive most likely a small SSD as your main drive meant for Windows to boot faster with not much space left which is common. Do you have a bigger second internal drive? Probably not an SSD but you would have more space.
If you do get another drive eventually, ignore the people who suggest putting your game onto an external drive if you can avoid it. Why? External drives are not going to be as fast as internal drives. You will notice a dip in performance with the game installed on an external drive.
edit to add: External drives are best used as storage devices for any extra photos or files not in use. I use mine for all of my backed up game stuff (screenshots, save files, recorded videos, copies of mods and CC)
