PostgreSQL distinguish between "identifiers" and 'literal'. Identifiers are schema, table, column's, .. names, literals are others. A attribute in hstore are not SQL identifiers. So you have to pass their names as literals. Operator "->" is only shortcut for function "fetchval(hstore, text)" with possibility be indexed.
select attributeValue->'CODE_MUN' from shapefile_feature
is internally transformed to (don't do this transformation by self!)
select fetchval(attributeValue, 'CODE_MUN') from shapefile_feature
on buggy example in transformed form, you can better understand to error message:
select fetchval(attributeValue, "CODE_MUN") from shapefile_feature
PostgreSQL tries to find column "CODE_MUN" in shapefile_feature, bacause used double quotes means identifiers (in case sensitive notation).