I saw many questions on SO with this error. But none were related to forms. I just hosted my application server after testing locally. I think it worked for a few mins but I
Considering that a LocalDb instance only intended for use in development. LocalDb is not available for production servers When you deploy the final result.
I think your connection string is pointing to a LocalDb instance and you need to take certain steps to turn it into the SQL Server database. It's not just a matter of copying an mdf file either. It will possibly differ from one hosting company to another, but typically, you need to create a backup of your existing database (a .bak file) and then restore that to the hosting company's SQL Server. You should ask them where you can find instructions on deploying your database into production.