SQLite does not support dropping or altering columns. However, there is a way to work around this by making changes at the table level: https://www.sqlite.org/lang_altertable.html
And more usefully for Alembic/Flask-Migrate users, Alembic’s batch_alter_table context manager lets you specify the changes in a natural way, and does a little “make new table – copy data – drop old table – rename new table” dance behind the scenes when using SQLite. See: http://alembic.zzzcomputing.com/en/latest/batch.html
So the upgrade() function in your migration file should contain something like:
with op.batch_alter_table('posts') as batch_op:
batch_op.drop_column('tags')
I’m afraid I don’t know why the error changed the second time you tried the upgrade.
As tkisme points out, you can also configure the EnvironmentContext.configure.render_as_batch flag in env.py so that autogenerated migration scripts will use batch_alter_table by default. See: http://alembic.zzzcomputing.com/en/latest/batch.html#batch-mode-with-autogenerate