Hi everyone,

I'm having trouble solving a set of pdes and keep encountering the following error:

Error, (in pdsolve/indexlex) invalid input: a string/name list is expected for sort method `string`

I have been trying to find out why I get the error or what the indexlex error is but am stumped... I cannot find anything on the Maplesoft or mapleprimes websites about it.

This is the code I am using which is giving the error: