SequelizeDatabaseError: cannot execute UPDATE in a read-only transaction
It can be caused by these reasons

1) you connected to a follower. Check it, using select pg_is_in_recovery(); – on master, it will return false:

test=# select pg_is_in_recovery();
 pg_is_in_recovery
-------------------
 f
(1 row)


Of course, you can run UPDATEs only on the master DB.

2) (I suppose this is not your case, but just for completeness I add it) You're working with master DB, but transaction or session was set to be "read only":

set session characteristics as transaction read only; 


or, inside transaction where UPDATE is being executed:

set transaction read only;