Rated 4 out of 5 stars
To use Send Later, no OpenPGP key can be associated to the account.
With OpenPGP key associated, draft is saved with encryption by the key.
This makes Send Later fail due to `Encryption compatibility issue`.
Probably Send Later cannot decrypt the scheduled mail by itself.
OpenPGP having been promoted as a standard feature of Thunderbird,
it would be nice if Send Later could handle this
(Though I know considering OpenPGP behavior would be too hard or perhaps impossible).
Edit on Thu Jun-17-2021 (Reply):
Got it. I'll wait for the Thunderbird update. Thank you!
Send Later can't handle this case because of a deficiency in Thunderbird. The Thunderbird development team is in the process of addressing this deficiency. Soon there will be a way to avoid this issue. See https://bugzilla.mozilla.org/show_bug.cgi?id=1681168.