the following script will create the missing website
IF NOT Exists (SELECT * FROM {databaseOwner}[{objectQualifier}Portals] WHERE PortalID = 0) BEGIN
SET IDENTITY_INSERT {databaseOwner}[{objectQualifier}Portals] ON;
INSERT INTO {databaseOwner}[{objectQualifier}Portals] ([PortalID], [AdministratorRoleId], [RegisteredRoleID]) Values(0, 0, 1);
SET IDENTITY_INSERT {databaseOwner}[{objectQualifier}Portals] OFF;
INSERT INTO {databaseOwner}[{objectQualifier}PortalLocalization] ([PortalID], CultureCode, PortalName) VALUES(0, N'en-US', N'dummy Portal');
SET IDENTITY_INSERT {databaseOwner}[{objectQualifier}Roles] ON;
INSERT INTO {databaseOwner}[{objectQualifier}Roles] ([RoleID], [PortalID], [RoleName], [BillingFrequency], [BillingPeriod], [TrialFrequency], [TrialPeriod]) Values(0, 0, N'Administrators', N'N', 1, N'N', 1);
INSERT INTO {databaseOwner}[{objectQualifier}Roles] ([RoleID], [PortalID], [RoleName], [BillingFrequency], [BillingPeriod], [TrialFrequency], [TrialPeriod]) Values(1, 0, N'Registered Users', N'N', 1, N'N', 1);
SET IDENTITY_INSERT {databaseOwner}[{objectQualifier}Roles] OFF;
INSERT INTO {databaseOwner}[{objectQualifier}PortalLanguages]([PortalID], [LanguageID]) SELECT 0, LanguageID FROM {databaseOwner}[{objectQualifier}Languages] WHERE CultureCode Like N'en-US';
END
You may remove website 0 after you upgraded to DNN 7.4.1