The error happens when you import an odoo14 db, if your odoo user doesn't have the appropriate permissions to create the extension then you will get this error.

To solve it, you can manually create the missing extension in the database using the postgres (superuser) user. Log in to the postgres db, connect to your imported db and run the following:

CREATE SCHEMA unaccent_schema;

CREATE EXTENSION unaccent WITH SCHEMA unaccent_schema;

COMMENT ON EXTENSION unaccent IS 'text search dictionary that removes accents';

CREATE FUNCTION public.unaccent(text) RETURNS text
    LANGUAGE sql IMMUTABLE
    AS $_$
        SELECT unaccent_schema.unaccent('unaccent_schema.unaccent', $1)
        $_$;

Restart your odoo server and you should be able to load the database successfully.