I associated a business account for an unrestricted external user and have provided both portal admin and customizer roles but still cannot customize on the portal and when
I have a case open with Acumatica on the same issue. The problem is the main project browser page is missing from the portal site map. Run the SQL statement below on the database and restart the Acumatica instance.
UPDATE 6/15/2018 - I ran into this issue again on another upgrade and my original script below didn't work because the Position field was wrong. I have adjusted it to insert into Position 1036 which now seems to work.
INSERT INTO PortalMap (CompanyID, Position, Title, Description, Url, Expanded, IsFolder, ScreenID, CompanyMask, NodeID, ParentID, CreatedByID, CreatedByScreenID, CreatedDateTime, LastModifiedByID, LastModifiedByScreenID, LastModifiedDateTime)
SELECT CompanyID, 1036, Title, Description, Url, Expanded, IsFolder, ScreenID, CompanyMask, NodeID, '84351BC9-BF6C-48B5-9DEA-F8207283B64A', CreatedByID, CreatedByScreenID, CreatedDateTime, LastModifiedByID, LastModifiedByScreenID, LastModifiedDateTime FROM SiteMap WHERE ScreenID = 'AU000000' AND CompanyID = 1