Problem Statement:

Disclaimer: 
This blog is only applicable for Business Central On-Premise Version. If you are stuck in the same issue, try this at your own risk. 

I had installed Business Central v14 (Spring 2019) On-Premise with LS Retail. In this, Business Central comes with a ready made setup to connect with VS Code. As I restored the DB given by LS Retail, I could no longer connect to VS Code. This could be a potential case of Running C/SIDE and AL Side-by-side or customising on top of LS Retail.

Despite all the configurations being correct in project's app.json and launch.json, I could no longer connect to Business Central, I couldn't download any Symbols and I couldn't publish any Extension to Business Central. 
App.json and Launch.json

I checked all BC Administration setup for Development and it seemed to be perfectly fine. I had Enabled Development Endpoint,  Development Endpoint Port was 7049Target was set to Internal, Enable Debugging was Enabled.
Development Endpoint - BC Administration
I checked my Permissions were set to SUPER, I had setup the WebServiceKey.
User Card
I had also checked my Developer License but still had the same thing 

This is where I noticed that, even after reverting back to the standard Demo BC(14-0) Database no longer helped. The only error I got was "The tenant 'default' is not accessible error".

I tried changing the AL Language Extension for VS Code guessing this could be a potential issue that BC v16 AL Language might not be able connect to BC v14.

Also, to ensure I created a new Server Instance and Web Server Instance, and tried connecting the same error "The tenant 'default' is not accessible error"
AL Project : The tenant 'default' is not accessible
I even tried a fresh install of BC but I simply couldn't figure out!!

Pre-requisites:


  • Microsoft Dynamics Business Central On-Premise DVD
  • AL Language Extension
  • VS Code
  • Basic Understanding of PowerShell

Books & References:

Resolution:

When it comes to a new Database, the Symbols for the DB used are not pre-generated.
AL Extension requires this symbol to be installed for symbols to be downloaded in the project for compilation process.
When these new symbols for different Database are generated, it also checks if the Data version is as per the latest version prior to publishing these new Symbols to the Database.

Importing the NAV-Admin module in PowerShell.
Import-Module NAV-Admin
Refresh the Command in Command Add-On in PowerShell

1. Checking if the Symbols are present for newly restored Database using Get-NAVAppInfo command:
Get-NAVAppInfo -ServerInstance <SERVERINSTANCENAME> -SymbolOnly
If you notice, you will see that the Application.App NAV-App is not installed.

2. You need to Generate the Application Symbol App:
i. From C/SIDE Development Environment, export all the objects into a single .txt file excluding the Codeunits >= 2000000001.

ii. Move this .txt file into a folder making sure that the folder has only this .txt exported of the objects

iii. Running the SymbolReferenceGenerator <Source Folder> <Output Package File Name> <Package Name> <Package Version> [<GenerateDevelopmentPackage>] command

Generate Symbols

3. Publish the NAV-App as Symbols Only:
This command will Publish Applications.app as Application symbols so that it can be downloaded inside AL Project.
Publish-NAVApp

4. Check if  NAV-App is installed:
This step will make sure that your NAVApp in compiled with the latest symbols and installed in BC.
Get-NAVAppInfo with Application.app

5. Check if AL Extension in VS Code can connect to Business Central:
After Restarting the Business Central Instance, I tried to connect to AL Project with Business Central Server.
AL Project: Symbols have been downloaded
Finally, I got it running and now I can deploy the Extension App to Business Central.

Conclusion:

Thus in this blog we saw how to enable customising using AL Extensions for a database that has already been customised by a 3rd Party.
I would like to share the acknowledgement of this blog to Mr. Manoj from Microsoft Support Bangalore for sharing his research and content and helping me in this issue.
I hope this helps!