Why welches is not correct?
"Mit" requires dative case, so it can only be "welchem" (masculine or neuter dative singular) or "welcher" (feminine dative singular).
Why not welchen or welchem?
Surely not welchen as that is for plurals?
Duolingo did not accept welchen...
"welchen" is accusative for masculine singular or dative for plural. Neither of them match here.