I got the same error. The trick is to do the installation step by step. 

1. Install an empty EPiServer CMS site
2. Install EPiServer Commerce on the same site
3. Install EPiServer Commerce Sample 

All using EPiServer Deployment Center

Please try the following:

• Use SQL Authentication (the user need to be in the SysAdmin role)
• Do not have any special characters in the database name, especially dash, period, quoutes (of any kind) or spaces
• Make sure the sql user names do not contain spaces or other special characters (as above)
• If you do not have SQL Server installed locally, please install the SQL Server Native Client and SQL Server Command Line Utility for the version you're using. (2008 R2 can be found here: http://www.microsoft.com/downloads/en/details.aspx?FamilyID=CEB4346F-657F-4D28-83F5-AAE0C5C83D52)
• Make sure the sqlcmd.exe file is in the system path. You might have to add it yourself and reboot in order for this to work.

Best Regards,

Steve Celius

I tried all of the above steps usin the sa user with SQL 2008 installed locally on Windows Server 2008 x86 SP2, but I still recieved the same error. sa is in the sysadmin role. The path to sqlcmd.exe was set and the server rebooted, but the same error.

Installing the CMS site, Commerce and Commerce Sample separately was the only way I could get around the problem and install without an error.

Regards

Dave Green

Any updates on this? I have tried installing everything separately. However I keep receiving the same error:

Error - The WriteObject and WriteError methods cannot be called from outside the overrides of the BeginProcessing, ProcessRecord, and EndProcessing methods, and only from that same thread. Validate that the cmdlet makes these calls correctly, or please contact Microsoft Support Services.

Same issue here, when upgrading.

First though, I received an error "Cannot evaluate a null expression"-like message. This was due to the fact that the installer doesn't support the configSource-attribute specified on the <appSettings>-element. Easily fixed, but still, that too needs to be fixed.