Import message base entry - error "Updates are not allowed"

Summary:

We have some custom message base entries for our AAQ page that will direct the customer to a company website. When we clone from prod to a test site, we want these message base entries updated so that the links point to a development website instead. I created a csv file for each of the message base entries involved, updating the links and attempted to import the file. I'm getting "updates not allowed" for each row. I have included all of the mandatory columns: key, type, length and text. Any advice on what could be wrong?

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