[flask_migrate] Error: Can't locate revision identified by '...'

If you delete your /migrations folder, the version is still saved in the database. You must also delete this version in the database:

1. Delete alembic_version table from database using psql:

$ psql postgres
\connect <Database>
DROP TABLE alembic_version;
\q

2. If your target database is not up to date, you will also have to run:

$ flask db stamp head
$ flask db upgrade

3. If you need to delete the alembic_version table on Heroku, run the following:

$ herok psql
delete from alembic_version;