“God made humankind upright.” God’s original creation of humankind was perfect, but He gave Adam and Eve free will, which they used in disobedience to God and lost their upright status. Although “humankind” is singlar, it is a collective singular as can be see by the “but they search,” which is plural.
“schemes.” This word “schemes” is almost identical to “reasons” in Ecc. 7:25 and 7:27. God has made things with specific reasons, and the wise person looks for them, but sadly fallen humanity searches out its own schemes, and ignores what God has done. This word “schemes” is from the same root as the word in 2 Chronicles 26:15, where king Uzziah made cleverly designed war “devices” to throw rocks and arrows.