error: ALTER TYPE … ADD cannot run inside a transaction block

As it was mentioned above you can’t edit enum within transaction block. But you can create the new one. Here are the steps:

1. Change type from request_type to varchar for all columns/tables which use this type:

ALTER TABLE table_name
    ALTER COLUMN column_name TYPE VARCHAR(255);

2. Drop and create again request_type enum:

DROP TYPE IF EXISTS request_type;
CREATE TYPE request_type AS ENUM (
    'OLD_VALUE_1',
    'OLD_VALUE_2',
    'NEW_VALUE_1',
    'NEW_VALUE_2'
);

3. Revert type from varchar to request_type for all columns/tables (revert step one):

ALTER TABLE table_name
    ALTER COLUMN column_name TYPE request_type
    USING (column_name::request_type);