The documentation portal is undergoing maintenance. During this period, you can't create any new user credentials. If you do, you'll need to recreate the user credential after the maintenance period ends.
The seamless access users are intended to be APPS like schemas with all the permissions of the APPS schema. The sample scripts are intended as example scripts. There may be additional grants required and some of the example grants may not be required. Check the data dictionary for exact grants to your APPS schema.