Postgresql psql: FATAL: role "username" does not exist - postgresql

Question

I have looked over all the questions posted previously and surprisingly NONE of them could resolve my problem. Can someone pls help me with this issue. I can post the outputs of the commands if someone can really help me. I am on MacOS BigSur 11.6
Here are the outputs of the common commands I tried from previous questions.
Command: psql or psql -l
Output: psql: FATAL: role "harshit" does not exist
Command: sudo psql
Output: psql: FATAL: role "root" does not exist
Command: su - postgres
Output: After entering the password , it says su: Sorry
Command: ps ax | grep postgres
Output:
47592 ?? S 0:00.12 /usr/local/opt/postgresql#9.6/bin/postgres -D /usr/local/var/postgresql#9.6
47599 ?? Ss 0:00.21 postgres: checkpointer process
47600 ?? Ss 0:00.11 postgres: writer process
47601 ?? Ss 0:00.08 postgres: wal writer process
47602 ?? Ss 0:00.07 postgres: autovacuum launcher process
47603 ?? Ss 0:00.38 postgres: stats collector process
49153 ?? Ss 0:00.03 postgres: stark m3_india ::1(65520) idle
49710 s002 S+ 0:00.01 grep postgres
Command: initdb /usr/local/var/postgresql#9.6
Output:
The files belonging to this database system will be owned by user "harshit".
This user must also own the server process.
The database cluster will be initialized with locales
COLLATE: C
CTYPE: UTF-8
MESSAGES: C
MONETARY: C
NUMERIC: C
TIME: C
The default database encoding has accordingly been set to "UTF8".
initdb: could not find suitable text search configuration for locale "UTF-8"
The default text search configuration will be set to "simple".
Data page checksums are disabled.
initdb: directory "/usr/local/var/postgresql#9.6" exists but is not empty
If you want to create a new database system, either remove or empty
the directory "/usr/local/var/postgresql#9.6" or run initdb
with an argument other than "/usr/local/var/postgresql#9.6".
I thought after the above one, it shown user as "harshit" but still using this username, psql doesn't work. I can't remove the existing db as it has some useful information. Is there no way to access it now?
Edit:
Command: sudo -u postgres psql
Output:
sudo: unknown user: postgres
sudo: error initializing audit plugin sudoers_audit
Command: ps aux | grep postgres
Output:
harshit 52614 0.0 0.1 4490468 24964 ?? Ss 4:50PM 0:00.47 postgres: stark m3_india ::1(50059) idle
harshit 52593 0.0 0.1 4489636 16320 ?? Ss 4:49PM 0:00.33 postgres: stark m3_india ::1(50052) idle
harshit 52290 0.0 0.1 4488464 8688 ?? Ss 4:39PM 0:00.04 postgres: stark m3_india ::1(50004) idle
harshit 51265 0.0 0.0 4342116 1140 ?? Ss 3:49PM 0:01.64 postgres: stats collector process
harshit 51264 0.0 0.0 4487076 2504 ?? Ss 3:49PM 0:00.28 postgres: autovacuum launcher process
harshit 51262 0.0 0.0 4487012 4896 ?? Ss 3:49PM 0:00.15 postgres: wal writer process
harshit 51261 0.0 0.0 4487012 1812 ?? Ss 3:49PM 0:00.14 postgres: writer process
harshit 51260 0.0 0.1 4487140 14444 ?? Ss 3:49PM 0:00.45 postgres: checkpointer process
harshit 51217 0.0 0.1 4487304 14068 ?? S 3:49PM 0:00.30 /usr/local/opt/postgresql#9.6/bin/postgres -D /usr/local/var/postgresql#9.6
_postgres 49645 0.0 0.0 4334640 1136 ?? S 3:35PM 0:00.03 /usr/sbin/distnoted agent
harshit 53335 0.0 0.0 4268424 616 s002 R+ 5:26PM 0:00.00 grep postgres
Command: whoami
Output: harshit
Command: psql -U postgres
Output: psql: FATAL: role "postgres" does not exist
Command: psql -U stark
Output: psql: FATAL: database "stark" does not exist
Edit: Trying to create users
Command: sudo -u postgres createuser harshit -s with password "123" or sudo -u postgres psql -c "ALTER ROLE harshit WITH PASSWORD '123';" or sudo -u postgres createuser -s -i -d -r -l -w harshit or sudo -u postgres -i
Output:
sudo: unknown user: postgres
sudo: error initializing audit plugin sudoers_audit