When you define a second account schema, you first define a new account element (the COA) and then you add an accounting schema (with accounting schema dimensions, general ledger and defaults).
The necessary step to add the new schema to an existing client is going to schema account defaults and in process icon you start the add or copy accounts ant this process will copy the default accounts to BP Group, BP, Product category, product and all the required tables. But this process fails because some tables already defined some default accounts (not used yet in version 2.1) as NOT NULL in database constraint. To fix this problem you must remove the NOT NULL clause in the database definition. The tables and fields that must be modified are:
Table C_BP_GROUP
fields
v_liability_services_acct
unearnedrevenue_acct

Table C_CASHBOOK_ACCT
cb_asset_acct
cb_cashtransfer_acct
cb_differences_acct
cb_expense_acct
cb_receipt_acct

The command to do this change (for each field) is:
ALTER TABLE C_BP_GROUP
alter column v_liability_services_acct drop not null;