No Database Instance

Me fixed, I was providing wrong database name.

In my case, going from my local SQLEXPRESS to an internal staging SQL server... my connection string needed:
Integrated Security=True

Integrated Security is not necessarily required.  You can set up users in the DB, but the DB needs to be configured in its "SQL Server Authentication Modes" for Mixed authentication - SQL Server and Windows Authentication mode.   (Using just Windows authentication is considered a more secure choice.)

imarrero wrote:
this was my problem too .thanks

I am getting the same error for as no database instance in this case. I had checked the connection string in the settings.txt and its correct.

In this case, I am migrating the website from one server to another server so I had copied all required source file and database. I am not sure what causing the site to run.

If anyone may have similar situation before, please let me know.

There are some of summery solution that i have mention as below
=> Since i had moved a live cart to a test server, the settings in the database for that cart had force ssl and also force www before any url. Also under Store settings i still had the domain info for that site which was now difrent in the test server. Once i noticed this and updated the database it all started working.

=>  You try change settings.txt file:
DataConnectionString: Data Source=localhost/IP;Initial Catalog=DBName;Integrated Security=False;Persist Security Info=False;User ID=DBUser;Password=DBPass;MultipleActiveResultSets=True

=> In my case I need config application pool for nopcomerce

my connection string
DataProvider: sqlserver
DataConnectionString: Data Source=.;Initial Catalog=nopcommerce370;Integrated Security=True;Persist Security Info=False

I need set Application Pool Identity to user has access right to db
Check the image to details.

=>  Check also db permission.

=> try with
Data Source=9X.2XX.7X.XX\"yoursqlservenameinstance"
or
Data Source="yournamemachine"
or
Data Source="yournamemachine"\"yoursqlservenameinstance"