Cmdstan should also be on 2.26, so it should work.
The code looks good at first glance, so maybe it is just older version of rstan not recognizing sort_desc. However, it looks like you throw away some values for cvxy that you compute in the loop - i.e. cvxy[lx + 2] and above is never used, so if you don’t use it you may as well not compute it all and save some performance.