It does not work for me. I get the following message when I try to pin something: "Sorry, something's not quite right here. Please check that you are logged into Pinterest and try again." It doesn't seem to matter what website I'm trying to pin from. I'm using 66.0.5 on a 2017 MacBook Pro (10.14.4 Mojave). I added the extension today. (I had an older version of the extension that worked, but I deleted it.)
EDIT: I log in with email. I cleared cookies and tried logging in again, but still get the error message. I don't know what account switching is so don't know whether I'm using it.
EDIT: are you seeing the error after the image picker comes up, you see your boards, and you choose Save to pick one? Do you ever share your computer with another Pinterest user?
I'm testing on your exact setup -- had to restart to apply v66.0.5 -- and I can't reproduce what's happening for you. How are you signing in to Pinterest, please? (Facebook, e-mail, Google, something else?)
If you're switching accounts in one tab after you bring up the image picker in another tab, you're going to see that something's-not-quite-right problem; are you doing that? If not, sorry if these are obvious questions, but: are you sure you're signed in to Pinterest? Are you using account switching?
Would you mind clearing all cookies and trying again with a fresh login to Pinterest?