This operation may be partially successful. Please check the status. Reason: Invalid Items for MSKU with reason ITEM_BLOCKED_BY_GATING.
This error message indicates that you are submitting a listing item for an ASIN, but Amazon has restricted marketplace sellers from selling against this ASIN with particular product conditions.
Make sure the SKU you are listing has a condition that is allowed for the ASIN and product type to avoid this issue. By searching the ASIN in the "List a new product" module on Amazon, you may display the listing restrictions and see the permitted criteria for that particular ASIN.
To find out the permitted conditions for your SKU's product type, you need to get the Amazon Product Type for this SKU. First, click on the SKU in your Catalog, then select the Categorization Card and find the Amazon Product Type.  You can check the conditions allowed for that type.