After Accepting My Resignation Letter, Denying My F&f Payment

I have been working in a someone since last 7-8 months, due to hostile working environment I decided to resign and gave them my resignation letter, they accepted the same as well, now on the payment of my outstanding amount for the 18 days I worked there, they are saying that they will not give me the amount as I didn't gave them the notice period. I just want to ask that if they accepted my resignation then why they need a notice period, they never demanded the same before my resignation and now saying that I am guilty of not serving a notice period. (this should not be like this, otherwise every company can use this strategy to cease the payment of employee by saying they didn't have the notice period after accepting their resignation, either they shouldn't have accepted the resignation letter or should have demanded the notice period.) is there any way of coping with them?

Does your letter of appointment mention any clause of notice period?
Your letter of appointment, resignation as well as acceptance has to be looked into before proceeding further.
Technically, the company may adjust the notice pay amount in case of failure of serving notice pay.
Otherwise, you have to approach appropriate forum to recover the outstanding wages of 18 days (for that your nature of job, company details, etc. is required)

Yes, in the appointment agreement there is a need to serve a notice period. But if they didn't ask for the same at the time of my resignation and accepted my resignation, are not they doing wrong? Either they shouldn't have accepted it or should have told me to serve a notice period.

The appointment agreement letter tells what an employer can do and what an employee should do.
So if you are not coming from the date of joining for a week (say for an example) that means you are breaching the agreement not the company.
Similarly, accepting the resignation is the duty of employer so it does and not coming in the notice period (as per mutual agreement) is the beach of the agreement from the employee’s side. Because the day after the resignation is accepted the notice period starts automatically unless it is mentioned in the agreement with any subsidiary clause.