I'll try and summarize and clarify everything, this should answer the butcher knife issue, as well as any other similar things at this point in the game:

The first "tier" of metalworking in Valheim is the arguably most complicated. It's the "Bronze Age." Bronze is made from Tin and Copper at the forge. There are very few things you can actually make with JUST tin or JUST copper.

To get started:
- You need a smelter and a kiln - it sounds like you have that covered if you're already holding tin bars.
- You need some Copper Bars. Copper is mined from large mossy boulders in the Black Forest. Some boulders are stone, some are copper, they look the same except copper has yellowish veins in it. Also, copper deposits say "copper" when you mouse over them (as a side note, the copper boulders are GIGANTIC and extend a good ways down and out from what you see on the surface....if you carefully mine out the entire edge of the rock so you don't miss any, you can get a TON of copper...you'll need it). Smelt the copper ore in the smelter for copper bars.
- You need to make a forge with those copper bars (and some coal and wood, but the bars will unlock the recipe for you). That's the crafting table for all metalworking stuff. If it uses metal bars, it's made at the forge.

(for a butcher knife, once you get here, you can now easily make the knife at the forge, however, most of the recipes in this tier of the game require one more step.)

- Make Bronze. Each 1 bronze bar requires 2 copper bars and 1 tin, and recipes use PLENTY of bronze....So you'll need a LOT of metal. Start tagging those copper deposits on your map because you'll likely be making several trips.
- I suggest your first item crafted from bronze be the axe, as that will let you chop birch and oak trees, which gives you access to "fine wood," which, along with copper, bronze, tin, and core wood, makes the last of the new crafting materials at this level (you'll need a ton of fine wood too for comfort objects, boats, portals, and some weapons)