kdb/q errors are famously terse. You (usually) receive a one word error message, with none of the extra 'help' seen in error messages from other languages.
This is fine when you are getting a 'type' error on 1+`a, but when you get a 'type' error on a more complicated operation it can be difficult to figure out why.Â
The code.kx errors article covers all of the errors with a basic description - the table below is an attempt to cover some of the less obvious examples.