Reformed types these days like to go around saying they believe in "Sola Scriptura", not "Solo Scriptura". By this, they mean that Scripture is our sole (and final) authority, but not that we read Scripture in a vacuum, by itself, stripped off from all pretense of reason, historical study, theological tradition, and so on. This is very true, and good theology. Unfortunately, it's terrible Latin.
Scriptura is a singular noun, feminine, meaning "writing", or more appropriately "Scripture", in the sense that we use the word to refer to the collection of Holy Writings in the Bible.
As a noun, it requires any modifying adjective to match its case, number, and gender.
Sola is a singular, feminine adjective, which matches Scriptura. Whether we take them as nominatives, ie. Scripture-alone as a concept, or as ablatives, ie. "by scripture alone", it works.
solo doesn't work. It could be a Latin word, but it doesn't fit scriptura. At best it's an English import, as we use the word 'solo'. Perhaps I've missed something, it's quite possible, but the Latin just seems bad to me. And there's no excuse for that.