Sorry that happened! Unfortunately, because the game offers so many options and ways of doing things out of any set order...it seems that certain things can sometimes "go missing". Especially later in the game. If you're sure you went to the right spot, but the trousers aren't there, there might be nothing you can do.
If you have the GOTY Edition on PC, you can transfer your saved game, use the console to create the trousers in your inventory, save again, then transfer it back to the Switch. That should work. If you don't have it yourself, maybe bug a friend that has it? You need a 100% vanilla installation of GOTY on PC (except for the console mod) and, like, 5 minutes.
Alternatively, check your inventory filters. It's possible the option is simply hidden.
Lastly, there was a similar thread about this, but I can't find it. Someone there had a similar issue. For whatever reason, it revolved around the dialogue with the lady-smith in Crow's Perch not having advanced far enough. They went to her, had a quick chat, then accessed the crafting menu again and it worked. (Sorry, I don't remember exactly what they did. If I can find that thread again, I'll let you know.)
