Notational Velocity is my favourite note taking app for little bits of information you’d like to save anywhere then be able to recall later.
Notational Velocity sync broke in October 2018 when the sync engine they relied on (Simplenote) changed the domain of their notes server. Over at GitHub there’s been a fix, but it hasn’t been distributed yet.
In the meantime, if you love Notational Velocity and can’t live without it, you can use NVAlt which is basically the same program but it’s been updated so not as to have the problem. It even uses exactly the same database.
You can download NVAlt from here: https://brettterpstra.com/projects/nvalt/