Is Security Update 2020-002 still available?
Direct link: https://updates.cdn-apple.com/2020/macos/061-90747-20200319-5ded2636-188e-428b-90e6-2ed1507f6f46/SecUpd2020-002HighSierra.dmg

in case it doesn't work, it's due to the key in between. Try the next link. 
Download page: https://support.apple.com/downloads/macos