It only has a total of 8GB to start with - then Linux and Android, and a bunch of both Android apps and carrier bloat gets added to it - so there's not much space to put anything on these days. An 8GB phone was great back in 2013, but today it can be used for a phone, contacts, email and maybe a couple of apps and photos and that's about it.
If you have things on it that aren't apps (documents, videos, pictures, music, etc.), add an SD card and move all of that to the card (copy, then paste, don't use Move) to free up a little space, but add a few apps and you're going to be out of space again.