Database error: attempt to write a readonly database SQLITE PYTHON

I've read in-numerous post on this, but with no luck. This is my directory structure.

|__ /var/www/username
|__ ├── main_project
│       ├── db.py
│       └── tasks.py
|       |__ some_file.py
└── |__ database_name.db

Following were the steps I studied on different post and tried the same :-

cd /var/www/username
chown www-data .
chown www-data database_name.db

and I still get the same error attempt to write a readonly database while inserting a new record which is a celery task.

I did chmod 777 which was unsafe , but then error turned to unable to open database file.

On all the posts, I've seen only these 2 steps, nothing else, but still receiving the same error. How can I solve this ??

On the other hand, when I run the script (manually) some_file.py which has DB insert queries, it works fine. What abnormal behaviour is this?

It was working fine from past 14-15 days. And suddenly I got this error.