A new user (called a schema in Oracle) is created using the CREATE USER SQL command. You need to be signed in as the SYS schema/user or as a schema that has been granted the rights to create schemas.
12c database comes in two basic flavours. Container database (containing pluggable databases). Standard database. If connected to a container database, you cannot create standard user schemas - you need to be connected to a pluggable database.
Also, your question has no relevance to either the SQL or PL/SQL languages - the subject matter of this forum. Please repost your question to a more appropriate forum dealing with SQL-Developer issues.
Moved to the SQL Developer from from SQL & PLSQL