What happened? 
For developers linking Merchant Center  accounts to Google Ads using Google Ads API v14 , there was an issue from 04:00 PST Monday, Feb 19  to 08:00 PST Tuesday, Feb 20  that caused some requests to the MutateMerchantCenterLink  method of MerchantCenterLinkService  to return MutateError.RESOURCE_NOT_FOUND . Also calls to ListMerchantCenterLinks  may not have returned some MerchantCenterLinks  with link status PENDING .
What should I do? 
If you use Google Ads API v14  to link Merchant Center accounts to Google Ads accounts using MerchantCenterLinkService , you should check to see if you have accounts that have pending link requests during the period of time described above. This issue has now been resolved and you may retry the failed calls  to complete the linking of Merchant Center accounts. Also check ListMerchantCenterLinks  for accounts with link status PENDING . Some of these link requests may not have previously been returned during that time period.
How to get help 
If you have any questions or need help, check out the Google Ads API support page  for options.