Try this:
Verify that profile schema is correct. If profile schema is incorrect then import profile schema again.
After importing schema, modify Sitecore Commerce 8.0 powered by Commerce Server Profile Schema for Sitecore Integration as described here.
I have found that profile database schema was not correct and u_external_id column was missing in UserObject table of profile database.
Make the following changes in UserObject table of profile database:
Add column: u_external_id NVARCHAR(256) NOT NULL
IF NOT EXISTS (SELECT * FROM INFORMATION_SCHEMA.COLUMNS WHERE TABLE_NAME = 'userobject' AND COLUMN_NAME = 'u_external_id')
BEGIN
ALTER TABLE dbo.UserObject ADD u_external_id nvarchar(256) NOT NULL
END
GO
Create Index on: u_external_id, UNIQUE NONCLUSTERED INDEX and ASC
IF EXISTS (SELECT * FROM sys.indexes WHERE object_id = OBJECT_ID(N'[dbo].[UserObject]') AND name = N'IX_UserObject_ExternalId')
DROP INDEX [IX_UserObject_ExternalId] ON [dbo].[UserObject]
GO
IF NOT EXISTS (SELECT * FROM sys.indexes WHERE object_id = OBJECT_ID(N'[dbo].[UserObject]') AND name = N'IX_UserObject_ExternalId')
CREATE UNIQUE NONCLUSTERED INDEX [IX_UserObject_ExternalId] ON [dbo].[UserObject]
(
[u_external_id] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, SORT_IN_TEMPDB = OFF, IGNORE_DUP_KEY = OFF, DROP_EXISTING = OFF, ONLINE = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
GO
Update Sitecore Commerce 8.0 powered by Commerce Server Profile Schema. Add a new GeneralInfo.ExternalId profile property that maps to the external id column, it must be required and unique.
Refresh the cache and reload the register account page. You should no longer get the above error message and should be able to create new user account in demo storefront website.
