If you are getting the following error message, when trying to save an existing product, or creating a new one:

you have to do the following.

• Review if you have a named “default” stock source on your admin. Under Stores > Inventory > Sources. If you don’t have this one, let’s create it.
• If the problem persist, try running the following commands below:
  
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  rm -rf generated/* php bin/magento indexer:reset php bin/magento indexer:reindex php bin/magento cache:clean

• If problem persist, you can also try changing your indexes from “update on save” to “update by schedule” and try. This can be a solution or a workaround, but at least you will have a north to see were your problem is.