Plant your stuff, harvest, upgrade your cauldron.

Must've changed in Hearth and Home. I was confused too.

Turnip seeds are found in the swamps. They aren't as common as carrot seeds but the flower is a nice bright yellow in the gray and black swamp so they aren't hard to see though you may have trouble finding the plant. When you get the seeds it opens the turnip plant in your recipes. Then plant the turnip seeds in your garden and when you harvest the 3 turnips you unlock the seed turnip recipe. Turn your 3 turnips into seeds, harvest and plant again, then use 3 of the turnips to make the spice rack and plant the rest of the turnips for seeds.

Ah so it was that then?
As the above have told you thats what you'll need heres the wiki entry if you get stuck or wanna precheck mats etc>
https://valheim.fandom.com/wiki/Spice_rack
Good luck!

What level is your cauldron?

how do I upgrade it?

You need the spice rack, I think it unlocks after finding Turnips.

And where do I find Turnips?