Did you generate the migration for it?

For example, here is mine adding that function:

defmodule Core.Repo.Migrations.InstallAshFunctionsExtension420240925162123 do
  @moduledoc """
  Installs any extensions that are mentioned in the repo's `installed_extensions/0` callback

  This file was autogenerated with `mix ash_postgres.generate_migrations`
  """

  use Ecto.Migration

  def up do
    execute("""
    CREATE OR REPLACE FUNCTION uuid_generate_v7()
    RETURNS UUID
    AS $$
    DECLARE
      timestamp    TIMESTAMPTZ;
      microseconds INT;
    BEGIN
      timestamp    = clock_timestamp();
      microseconds = (cast(extract(microseconds FROM timestamp)::INT - (floor(extract(milliseconds FROM timestamp))::INT * 1000) AS DOUBLE PRECISION) * 4.096)::INT;

      RETURN encode(
        set_byte(
          set_byte(
            overlay(uuid_send(gen_random_uuid()) placing substring(int8send(floor(extract(epoch FROM timestamp) * 1000)::BIGINT) FROM 3) FROM 1 FOR 6
          ),
          6, (b'0111' || (microseconds >> 8)::bit(4))::bit(8)::int
        ),
        7, microseconds::bit(8)::int
      ),
      'hex')::UUID;
    END
    $$
    LANGUAGE PLPGSQL
    VOLATILE;
    """)

    execute("""
    CREATE OR REPLACE FUNCTION timestamp_from_uuid_v7(_uuid uuid)
    RETURNS TIMESTAMP WITHOUT TIME ZONE
    AS $$
      SELECT to_timestamp(('x0000' || substr(_uuid::TEXT, 1, 8) || substr(_uuid::TEXT, 10, 4))::BIT(64)::BIGINT::NUMERIC / 1000);
    $$
    LANGUAGE SQL
    IMMUTABLE PARALLEL SAFE STRICT;
    """)
  end

  def down do
    # Uncomment this if you actually want to uninstall the extensions
    # when this migration is rolled back:
    execute("DROP FUNCTION IF EXISTS uuid_generate_v7(), timestamp_from_uuid_v7(uuid)")
  end
end