Hey @Chaitany patel7,
You cannot buy F&O scrips with the product type CNC. You need to select "Margin" as the product type in order to take delivery in F&O. "CNC" is only for Equity.
By selecting Margin as the product type, your positions will not get squared-off the same day, and you will be able to carry forward it to the next trading day.
Also, if you're taking an Equity position with the product type as CNC and want to sell it during the same trading day or next trading day, you can do so as BTST is allowed in Fyers.
Hope this clarifies!