I don't know your computer configuration, but some new computers come with Windows installed on a very small SSD (for speed) combined with a much larger spinning hard drive for storage.
So check your file manager (Windows Explorer) for what your disks are. If your operating system is on a small SSD and you have plenty of space on a 2nd drive, you may have to change the location of your Steam folder to be on the larger hard drive.
Here's a Steam page telling how to move your Steam folder
https://support.steampowered.com/kb_article.php?ref=7418-YUBN-8129