nextval() takes a sequence name. not a function like uuid_generate_v4().
if you insist on using UUID (very slow to generate, very bulky), then try...
INSERT INTO junk.wm_260_billables2 (account_id, code, info) SELECT account_id, code || '_' || uuid_generate_v4(), info FROM junk.wm_260_billables1;--
john r pierce, recycling bits in santa cruz