"Error getting LWA Access Token on Marketplace" occurs while creating a shipment. 

Upon reviewing your marketplace settings, it appears that the API is connected.

Solution:
To address this issue, you'll need to re-authorize SellerChain. 

Step 1: Navigate to your Amazon Seller Central account and go to the "Manage my Apps" section. You'll find this section leading you to the page displayed in the screenshot below. Scroll down until you locate Seller Chain, then click on "Re/authorize".

Step 2: Open a new tab and sign into Seller Chain. (Avoid signing in when redirected from Amazon, as it won't work.) Once signed in, navigate to your manage marketplaces page. Press "disconnect marketplace" followed by "connect" again.

Once all these steps are completed, you won't encounter this error anymore.