When you change a schema using Prisma, Vercel won't automatically regenerate the Prisma client until you add a postinstall command to package.json.
"scripts": {
"dev": "next dev",
"build": "next build",
"start": "next start",
"lint": "next lint",
"generate": "prisma generate", // add these two
"postinstall": "npm run generate"
},
Solution found here.