1

Add 'build' script to ./package.json

 {
   "scripts": {
+    "build": "email build"
   }
 }
2

Change 'Framework Preset' on Vercel's project settings to Next.js

You also need to add “next” on devDependencies to work properly:
 {
   "devDependencies": {
+    "next": "*",
   }
 }
This is a limitation on Vercel’s Next Framework Preset.
3

Change 'Output Directory' to .react-email/.next

In the end, your settings should look like this:
Proper Vercel settings