Invalid item for MSKU XX_YOUR_SKU_XXX with reason MISSING_DIMENSIONS
This error occurred because the product dimensions are missing from the shipping details.
Usually when an ASIN is created on Seller Central dimensions are input as well; but sometimes they're overlooked.
In SellerChamp open the item to 'Edit' and click 'Advanced View' in the top right corner after it pops up; that will give you access to the Shipping Details at the bottom.
If one or both of these fields is empty then enter the dimensions for the items and click 'Save Item'. That should take care of the error. You may need to go to the Functions to the right, the little box with three marks, and click 'Mark as Valid'. That should take care of this error.
If that doesn't solve the issue, login into Amazon SellerCentral and fill in the package dimensions under the "Other Details" tab for the product.