I am trying to add Commerce 7.5 to an existing EPiServer 7.5 CMS site.

I use the deployment center and setup the database and Commerce Manager site, and start the install.

It almost immedately fails with: Missing required parameter $properties.Site.ExternalFilesPath that should point to the appdata folder

Any ideas what I am missing here?

The full log for the install, as far as I get.

Begin-EPiBulkInstall transaction of Commerce installation
Started a transacted install
Installing Common Framework features to site 
C:\Users\andreasj\Projects\episite\site\wwwroot
 
Install Common Framework on selected site
productPath: C:\Program Files (x86)\EPiServer\Framework\7.5.394.2\
installedVersion: 7.6.3.0
frameworkVersion: 7.5.394.2

An unhandled error has occured:
Missing required parameter $properties.Site.ExternalFilesPath that should point
 to the appdata folder.
When executing

At C:\Program Files (x86)\EPiServer\Framework\7.5.394.2\Install\System Scripts\
Install Site (SqlServer).ps1:202 char:14
+         throw <><><><  (New-Object ArgumentException('Missing required parameter
 $properties.Site.ExternalFilesPath that should point to the appdata folder.'))

True
=
Get-EPiIsBulkInstalling
An error has occured and the transaction will be rolled back
Rolledback
===in trapping ex ===
An unhandled error has occured:
Missing required parameter $properties.Site.ExternalFilesPath that should point
 to the appdata folder.
When executing

At C:\Program Files (x86)\EPiServer\Framework\7.5.394.2\Install\System Scripts\
Install Site (SqlServer).ps1:202 char:14
+         throw <><><><  (New-Object ArgumentException('Missing required parameter
 $properties.Site.ExternalFilesPath that should point to the appdata folder.'))
inTransaction = False
Deleting Commerce Manager site folder C:\EPiServer\episitecommerce\wwwroot
Deleting Commerce Manager ExternalFiles C:\EPiServer\episitecommerce\eCommerceFra
mework\5.3\
Deleting Commerce Manager database 
episitecommerce
Started a transacted install
The database episitecommerce was not deleted because it did not exist
Entering commit phase
Committed

At C:\Program Files (x86)\EPiServer\Framework\7.5.394.2\Install\System Scripts\Install Site (SqlServer).ps1:202 char:14
+         throw <><><><  (New-Object ArgumentException('Missing required parameter $properties.Site.ExternalFilesPath that should point to the appdata folder.'))
System.ArgumentException: Missing required parameter $properties.Site.ExternalFilesPath that should point to the appdata folder.