Amazon does not always accept dimensions entered through InventoryLab or Seller Central in a timely manner.
If you have entered dimensions and they are not being accepted, you are able to submit your dimensions a different way that is not at the mercy of the Amazon API (the system 3rd parties use to communicate with Amazon).
If you have entered dimensions through InventoryLab and they are not being accepted after 15 minutes, there are 2 options to quickly get you back on track.
1. Add the MSKU to a shipment plan in Seller Central. This is done by either adding the product to your inventory or locating it and using the send/replenish action
2. Amazon will prompt for dimensions. Enter the dimensions.
3. Return to InventoryLab and list the item as normal.
4. After the shipments have been created, it is safe to delete the shipping plan entry. You should not delete the shipment plan or shipment plan entry until the shipments are created in Seller Central.
Add the item to your Amazon inventory and save it.
Once the item is saved, in Seller Central, click on Inventory and select Manage Inventory from the dropdown box
Next, click the Edit button next to the item
Click the More Details tab and enter the Item Dimensions towards the top of the page
Scroll down and enter the Package Dimensions (this one is the most important)
Finally, scroll all the way down and click Save and Finish
Copy the MSKU if you want to use it for your listing in IL.
Return to IL and attempt to list the item again.
If the dimensions still aren't being accepted, try opening the Seller Central pages in a different browser and trying again. If it still isn't accepted, reach out to Amazon Seller Support's FBA team for assistance.