There are several issues with your code: Firstly, input values are treated as strings by default, so you won't be able to perform arithmetic operations on them without converting them using parseInt (for integers) or parseFloat (for decimals). Secondly, if you want to retrieve text from an input element, use ".value" instead of ".innerText". Lastly, when displaying a product price like 12999, ensure it is in its full numerical format and only show commas or periods based on the desired representation.

On the HTML side, your code should resemble this:

<div class="container">
        <h3>Solar terrace light with inbuilt panel</h3>
        <div class="row">
            <div class="col-md-6">
                <img src="https://s3.ap-south-1.amazonaws.com/rzp-prod-merchant-assets/payment-link/description/pr11_hq0cy7e5cmwbzw"
                    class="w-75">
            </div>
            <div class="col-md-6 mt-3">
                <div class="card p-4">
                    <h4>Payment Details</h4>
                    <div class="row mt-2">
                        <div class="col-3">
                            <p>Qty.</p>
                        </div>
                        <div class="col-9">
                            <div class="quantity">
                                <button class="btn minus-btn disabled" type="button">-</button>
                                <input type="text" id="quantity" value="1">
                                <button class="btn plus-btn" type="button">+</button>
                            </div>

                            <!--will calculate price---->

                        </div>
                    </div>
                    <div class="row mt-2">
                        <div class="col-3">
                            <p>Name</p>

                        </div>
                        <div class="col-9">
                            <input type="text" class="w-100">

                        </div>
                    </div>
                    <div class="row">
                        <div class="col-3">
                            <p>Email</p>

                        </div>
                        <div class="col-9">
                            <input type="text" class="w-100">

                        </div>
                    </div>
                    <div class="row">
                        <div class="col-3">
                            <p>Phone</p>

                        </div>
                        <div class="col-9">
                            <input type="text" class="w-100">

                        </div>
                    </div>
                    <div class="row">
                        <div class="col-3">
                            <p>Price</p>

                        </div>
                        <div class="col-9">
                            <p class="total-price">
                                <span><i class="fas fa-rupee-sign" style="color: black;"></i></span>
                                <input id="price" class="text-dark" value="12.999"> 
                            </p>
                        </div>
                    </div>
                    <button class="btn btn-primary">Buy Now</button>
                </div>
            </div>
        </div>
    </div>

And for the JavaScript part:

document.querySelector(".minus-btn").setAttribute("disabled", "disabled");

        //variable for value count incrementing/decrementing
        var valueCount

        //setting price value in a variable
        var price = document.getElementById("price").value;
        console.log(price);

        //function for calculating total price
        function priceTotal() {
            var total = parseFloat(valueCount) * parseFloat(price);
            document.getElementById("price").value = total
        }

        //plus button functionality
        document.querySelector(".plus-btn").addEventListener("click", function () {
            //getting input value
            valueCount = document.getElementById("quantity").value;

            //increasing input value by 1
            valueCount++;

            //setting incremented input value
            document.getElementById("quantity").value = valueCount;

            if (valueCount > 1) {
                document.querySelector(".minus-btn").removeAttribute("disabled");
                document.querySelector(".minus-btn").classList.remove("disabled")
            }

            //calling price function
            priceTotal()
        })

        //minus button functionality
        document.querySelector(".minus-btn").addEventListener("click", function () {
            //getting input value
            valueCount = document.getElementById("quantity").value;

            //decreasing input value by 1
            valueCount--;

            //setting decremented input value
            document.getElementById("quantity").value = valueCount

            if (valueCount == 1) {
                document.querySelector(".minus-btn").setAttribute("disabled", "disabled")
            }

            //calling price function
            priceTotal()
        })

To update the content using "innerHTML", replace it with "value" and eliminate the "," (comma) from the price. It's as simple as that!