If you are talking about the Orange Plastic Frame that comes with each Ryzen CPU, It is best to open a AMD WARRANTY REQUEST from here: AMD Warranty Request Form . Several Users in the past have asked the same question. Seems like that Orange Plastic Frame is not sold separately.
If you talking about the actual CPU Cooler itself that has some sort of installation frame, try contacting the CPU Cooler manufacturer and see if they will sell or give you one.
If that adapter came with your Pre-built Computer and is part of your CPU Cooler, It seems like the Store is required to give you one or replace it if missing. Otherwise I would send the computer back if they don't give you one if the computer supposed to have one included.
As you can tell, You question is a little bit confusing so I just covered all bases as best as I could. Need to give more information on your computer, especially make & model of CPU Cooler and CPU.
This is the adapter that they kept instead of shipping along with the computer.
When you bought the Pre-Built Computer, was the CPU Liquid Cooler installed or did you need to install it yourself?
If the CPU liquid Cooler wasn't installed but was provided as part of the computer purchase by the retailer, They have to make certain your computer has all the parts needed to finish completing the Pre-built computer including all parts for the CPU Liquid Cooler if it came with the computer you purchased.
Since you can purchase the part, separately, I imagine you can order and pay for the part and then have the Retailer reimburse you for the money you spent.
BUT I don't know the terms of sales for your Pre-Built Computer. So, if the Retailer doesn't want to honor the request to replace the missing part you will need to take it up with the Retailer or put a complaint in with a Consumer type Agency or local BBS (Better Business) that can investigate what happened including hard evidence that the Retailer refused to replace the missing part and that you needed to purchase the part from the CPU Liquid Cooler manufacturer to get your computer working.
Everything I mentioned above is just my own opinion. Unfortunately, That bracket is not part of the Ryzen CPU thus AMD has no responsibility to replace it.
If you bought it in Europe, There are European Agencies that can help you with your case against the Retailer. If you bought it in USA, you can put in complaints either to a Consumer Agency or BBS or the Retailer Corporate offices or even the Attorney General of your State.
But for a $8.00 bracket, I don't think it is worth all that trouble. Putting in complaints is not difficult and doesn't cost anything. It is your time spent.
You better hope your PC doesn't have any problems in the future. Seems like the Retailer is a store I wouldn't do business with any more in the future.