This article talks about commonly known pool creation errors. Personally I have had these errors number of times during setup of LCS or OCS infrastructure.

Error 1:

An error occurred during the pool backend detection. Pool backend discovery failed. Failed to start SQL service or invalid SQL instance. This can happen due to network or credential problems. Please make sure network is good and logon has sufficient permissions. Click 'Cancel' to close the wizard. Click 'Try Again' to go back to the previous page and enter a different pool backend instance. Click 'Continue' to move forward.

Solution:  http://support.microsoft.com/?kbid=961946

Error 2:

An error occurred during pool backend detection: Pool backend discovery failed. Failed to open SQL database.

Solution: http://support.microsoft.com/?kbid=968979

Error 3:

Pool backend discovery failed. [hr=0xC3EC783C]

Click 'Cancel' to close the wizard.
Click 'Try Again' to go back to the previous page and enter a different pool
backend instance.
Click 'Continue' to move forward
 

Wizard log gives following error

Failure [0xC3EC783C] An error occurred while running DB script
 

Solution: Instead of running pool creation wizard from the Front End server, run it from the SQL server (Backend). After successfully creating pool, complete further installation process on the Front End server.

Error 4:

Pool Backend Discovery Failed. Failed to Start SQL service or invalid SQL instance

Solution: 

Make sure that SQL Server Browser Service is running on the SQL Server. This service handles redirect of clients to connect to the Named SQL instance.

Ensure that windows firewall is turned off on the machine where we are creating pool.