ERROR: text search configuration name "english" must be schema-qualified
Double quotes are for case sensitive identifiers or one's that have whitespace in them. Single quotes are for strings.

Try this:

UPDATE "EMAIL" SET PostText = to_tsvector('english', 'TEXT');


You can also remove the quotes around "EMAIL" for a case insensitive match on the field name as well:

UPDATE email SET PostText = to_tsvector('english', 'TEXT');