Short answer
Amazon only counts a shipment toward your Valid Tracking Rate (VTR) if the service is fully tracked. Royal Mail 24, Royal Mail 48, 1st Class and 2nd Class are not fully tracked. They only confirm delivery, and parcels that aren't scanned at the door show as "No valid tracking", even when you buy them through Amazon.
To fix it, ship Amazon orders with Royal Mail Tracked 24, Tracked 48, or Special Delivery Guaranteed. Buying those labels through Amazon Shipping also gives you the strongest protection against A-to-z claims, because Amazon knows exactly how the order was shipped.
How to set it up in SKULabs
Connect Amazon Shipping: go to Settings > Shipping > Carrier Accounts, click Add carrier, and choose Amazon Shipping V2. See Using Amazon Shipping with SKULabs.
If you ship with your own Royal Mail account instead, connect it as described in How to connect Royal Mail account to SKULabs.
Update the shipping presets and shipping rules you use for Amazon orders so they pick a Tracked 24 or Tracked 48 service instead of Royal Mail 24/48 or 1st/2nd Class.
Fetch rates on a test Amazon order to confirm the tracked service appears.
If you confirm shipment manually on Amazon
Pick Royal Mail as the carrier and the exact service (for example Royal Mail Tracked 48), not a generic option like "2nd Class". Untracked services still count against your VTR.
For Amazon's current rules, see Valid Tracking Rate in Seller Central Help.