SAP SD User Ticket:

Confirm quantity date is next day.

Stock is available in respective plant and storage location. (Unrestricted stock)

I checked with different Qty of sales order.

What could be the reason?

The reason could be there should be processing times configured in the system like..pick/pack times, transit time. Please check the shipping point and also the route for different lead times.
The other possibility could be holidays.
First check what the requested delivery date you have put in the order. Next check the lead times, for that you can go to the schedule line of the item and click on the Procurement tab.
Check in OVLZ ---> In Pick/pack time working days, any value is maintained there. For example, if 5 is given in this field, even if stock is available on the same day of order creation, system will allow to do PGI only after 5 days from the date of order created.
Also go to VOV8, select your sale order type and check the Field "Lead Time in Days".
 
I'm using service material in my sales order and I need the schedule line to confirm the quantity always in the actual day (not depending on the delivery date).
T. Code: VOV8
Select your Sales Doc. type and Double-Click.
In Tab: Shipping,
Check Box: Immediate Delivery (press F1 to read more about this functionality).
Note: There are few more settings that you may check:
1. T. Code: OVLZ
Field: Pick/pack time wrkdys whether you have maintained any value. It should have been blank 
2. T. Code: VOV8
Check how many days mentioned in Lead time in Days. If it is mentioned any days, remove it.
3. T. Code: OVZ9
Checked the Box: Check without RLT .
4. Check in material master MRP2 view how many days are maintained for the fields In-House production and GR Processing Time