Please explain SYSCS_UTIL.SYSCS_CREATE_USER() function
Read the secderby.pdf, page 41, "Configuring NATIVE authentication":

Use the SYSCS_UTIL.SYSCS_CREATE_USER system procedure to add credentials
for the Database Owner. Remember that the Database Owner is the user who
created the database.

I assume you created your DB without username/password or you tried to create a different user then the one when you created the DB.

Working example:
CONNECT 'jdbc:derby://localhost:1527/myDB;create=true;user=admin;password=admin123';
call SYSCS_UTIL.SYSCS_CREATE_USER('admin', 'admin123' );
You see in the above example the DBO (database owner) is 'admin' and for the very first SYSCS_CREATE_USER call, the user must be 'admin' too.

Not working example:
CONNECT 'jdbc:derby://localhost:1527/myBadDB;create=true;';
call SYSCS_UTIL.SYSCS_CREATE_USER('admin', 'admin123' );