When using Oracle 12c, immediately after ugprade or installation you need to follow the instructions in SAP Note 2157904 to address the following issue.
When you use Oracle version 12c, tables that were previously compressed under Oracle version 11g with the option "COMPRESS FOR OLTP" are incorrectly created with the option "COMPRESS" (= "ROW STORE COMPRESS BASIC") after the conversion.
As a result, when you try to add additional fields with "ALTER TABLE <tablename> ADD ( <fieldname> DEFAULT <default value> NOT NULL )", your attempt fails with the Oracle error ORA-39726 (Unsupported add / drop column operation on compressed tables).
The latter may occur when you import Support Packages, as well as when you manually add fields in transaction SE11.