Suppose when sales happening on specific SKU's. It's better to convey to customers a proper message on the product page
so the first step needs to tag the product with a specific name if you installed apps like https://matrixify.app/ will help to import easily with tags exchange. otherwise, you have to do with normal Shopify export & import.
on product-template.liquid or on some theme its product-default-template.liquid
add this below code
{% for tag in product.tags %}
{% if tag == "Flash Sale" or tag == "deal-ofthe-day"%}
<span>⚠️ Coupon Codes not Applicable, Already at Best Price.</span><br/>
{% endif %}
{% if tag == "sample"%}
<span><img class="limit2-svg-icon"> Limited Purchase Quantity</span><br>
<span>⚠️ Coupon Codes not applicable, Already at best price</span><br/>
{% endif %}
{% endfor %}
Suppose scenarios like when you want to add the message which is on offer, and without offer tag as a different message, here is the example
{% for tag in product.tags %}
{% if tag == "Freedom"%}
{% assign freedom_tag = tag %}
{% endif %}
{% endfor %}
{% if freedom_tag %}
Buy 4 Pay for 2 - Use code : <span style="border: 2px dashed #ebcc71; padding: 0 10px 0
10px;">FREEDOM</span><br/>
{% else %}
<span>⚠️ Buy 4 Pay for 2 Not Applicable</span><br/>
{% endif %}
Here is the scenario when you want to run offer freebies only for a few SKU's, and want to display a message with steps and option to add free product cart
{% if tag == "SOME-TAGS" %}
<img class="offer-svg-icon"> GET <a href="#" style="color:#F58148">IMMUNITY BOOSTER
</a>FREE ON PURCHASE OF WORTH ₹999. <br>
<span style="color:#456639"> HOW TO AVAIL OFFER: </span><br/> 1️⃣ Add <SOME TAGS>
Products Worth ₹999 & above.<br/>
2️⃣ Add Free Product to Cart
<form action="/cart/add" method="post" class="variants"
id="-4526022393969" data-id="product-actions-4526022393969" enctype="multipart/form-
data">
<input type="hidden" name="id" value="31858861539441">
<button type="submit" data-form-id="#-4526022393969" style="color: #fff;
background-color: #F6A300;
border-radius: 4px;">Click Here
</button>
</form>
3️⃣ Use Code <span class="coupon-box">FREE3FWD</span> at checkout.<br/> 4️⃣ Enjoy your
Order.
{% endif %}
In the above code