It tends to say that, even when there is one installed (see e.g. here). Have you already tried to reboot anyway?
If it boots cleanly, you’ll have to check whether the update did or did not complete. If it didn’t install, do you have any system modifications (xposed, root, etc) that could cause unexpected contents errors (or error 7) during the update?
If you’re on Fairphone Open OS and you end up in a boot loop or unstable system, do you have openGapps or similar installed? If so, did you remember to reinstall that after the update as outlined here?