perhaps try:
administrator
ADMINISTRATOR
Admin
admin
Case is definitely important.
If you’re still not sure, I just looked in /opt/couchbase/var/lib/couchbase/config/config.dat
I found my Administrator user and the password in clear text. If you’re sure about the password, search that file and find the string before that…
Also, later in that file appears to be some JSON, so you might be able to search on “userName”