M2E Pro requires a static, unique seller identifier to link with Walmart account. For that purpose, sellers have submitted their Consumer IDs to the Extension until recently.
On August 28, 2019, Walmart deprecated Digital Signature based Authentication on the US Marketplace and excluded Consumer ID data from Developer Center. New OAuth based Authentication implies using Client ID and Client Secret. These API credentials have a dynamic nature and need to be complemented with a static seller identifier to authorize M2E Pro on Walmart correctly.
To get around this situation, you can use either Consumer ID or Partner ID when authorizing M2E Pro on US Walmart Marketplace. Both are uniquely assigned to each seller at the partnership conclusion step.
If you registered your account before the new Walmart auth method came into force, you might have your Consumer ID. Otherwise, you can find your Partner ID in the reports downloaded from Seller Center: Manage Items > Download > Download Catalog.
Note: Your Consumer ID/ Partner ID must not be changed once it is obtained and sent to M2E Pro.
If you need to reauthorize Extension, please generate new Client ID and Client Secret for M2E Pro under Username > Oauth Access Token in Developer Center.