Follow the below mentioned step to generate MWS keys from your Amazon Seller Central account.
Step 1: Login to your Amazon Seller Central and go to menu Settings -> User Settings as shown in the below screenshot.
Step 2: Copy the Seller ID from this screen as show in the below screenshot. This is your Merchant ID to be entered in the APP. Now click on the View your credentials link as shown in the below screenshot.
If you do not see the View your credentials link as above setup, click on the button Register as a Developer at the same page as shown below.
Step 3: A popup will gets opened as shown in the below screenshot. You can copy your Access Key ID and Secret Key. You may need to click on link View to get the Secret Key.
Step 4: Log on to our APP – Xpert Importer in your Shopify admin and click on the option Link Amazon Account.
Step 5: Copy all the keys you have generated in above steps here.
If you see any validation error after Saving the keys, please recheck the keys. And contact customer support if the error still persist.