For my personal website I use monorepo approach. This allows me to make quick edits to my website while having everything in the same repository.
Somewhere in the last few months, I made a mistake that made it impossible to deploy my website using the default Vercel deployment process. After a while, I gave up and contacted Vercel's support to ask for help.
After a couple of days, I received a reply with the answer to my problem. When I read the email I instantly shook my head because I never thought of looking in the most logical file.
If you have trouble deploying your website using the normal deployment process take a look in
SUBDIR/.vercelignore. In my case I had the following files in there which made it impossible to deploy the application:
If this isn't the problem you might want to check the "Root Directory" settings in your project settings. This should be set to the appropriate path. Which in my case is