can add storage space in Mac desktop mac

Welcome to Apple Support Communities.

While you cannot add storage space, you do have a few options:

1. Manage Storage: You can manage storage in an attempt to gain some. Learn more: How to free up storage space on your Mac - Apple Support.
2. Use an external hard drive for storage: You could use a large external hard drive for storage. Learn more: Use external storage devices with Mac - Apple Support. If you go this route, get a high-quality, desktop-class, external hard drive, such as the LaCie 6TB d2 Professional Desktop Hard Drive - Apple Store.

-Jack