300-series boards need a current BIOS to run the newer CPU. If they ship you an older board it might not have the right one and require an older cpu just for the purpose of updating BIOS. Details:
Short Term Processor Loan "Boot Kit"
AMD will offer affected and qualified users (as described below) a AMD AM4 A-series processor including thermal solution for short term use ("boot kit") to perform the BIOS update on their motherboard.
This short-term processor loan or boot kit is offered through AMD warranty services and is available only for qualified users that have made a valid purchase of a 2nd Gen Ryzen™ Desktop Processor and are affected by this specific boot up issue.
This service is free of charge with the following condition:
Once you have submitted your claim for a processor loan boot kit, AMD will require pictures of your 2nd Gen Ryzen™ Desktop Processor and AMD Socket AM4 300-series motherboard, that clearly shows the model numbers and unique serial numbers as well as a copy of the purchase invoice(s) to authorize the request. Additionally, a summary or copy of communication with the motherboard manufacturer is requested to indicate why support from the Original Design Manufacturer (ODM) is not suitable.
Once the RMA request is approved, the AMD AM4 A-series processor will be sent with pre-paid return shipping. The processor is provided as a temporary loan to you for the sole purpose of updating the BIOS and must be returned within 10 business days of receipt. It is not necessary to return the provided thermal solution.
To obtain the boot kit, please carefully follow these instructions:
Go to the AMD online warranty claims page:
https://support.amd.com/en-us/warranty/rma
Fill in your full contact and product details (2nd Gen Ryzen Processor OPN number and serial number)
In the Problem Description field enter "Boot kit Required" (without quotes)
NOTE: The product must be a valid AMD processor with authentic OPN and serial number.
Once the claim is received and approved, confirmation details and boot kit instructions will be provided in a follow up email.