I've been hearing a lot about this message. (thanks for the screen cap)
If you have Office Pro, then that message is a flat out lie. For Office Pro, there is no limit to the number of times you can activate it. Check you license terms.
If you really have Office Pro Plus, then yes, there is a case where you can have a license type that allows you a limited number of activations.
We can't help with the problem. You will have to do as they advise at the very bottom, you will have to do a Phone Activation. So click on the "Phone activation" radio button then Next. It should display the phone number you need to use.
This is where you may see the next lie. You may see this message:
2017 01 20- “TELEPHONE ACTIVATION IS NO LONGER SUPPORTED FOR YOUR PRODUCT” ERROR WHEN ACTIVATING OFFICE
The problem started in the fall of 2016. In Jan 2017 a MS staffer provided the following excuse. But the problem still exists! MS oops’d, and just hasn’t bothered to fix the problem for much too long. Go to green link below to get the correct phone number for your specific geographic location
<snip https://answers.microsoft.com/en-us/msoffice/fo... >
Michelle V [MSFT]
We apologize for this inconvenience. We are currently fixing this bug in our Office products. In the meantime, you can call the numbers for your specific country/region listed in this article
(https://support.office.com/en-us/article/-Telep... ).
Please also note that this issue impacts Office non-subscribers only—those who purchased the product as a one-time purchase. For more information, see What’s the difference between Office 365 and Office 2016?
If this doesn’t solve the issue, please contact our support agents linked from the bottom of the same article.
Again, we apologize for this issue. We are working to get this resolved quickly and will update this thread once the fix is available.
Thanks,
Michelle V.
Office Team
</snip>
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When you phone in, don't reply to any of the prompts until they offer you the option of talking to a service rep. This is the one situation where you really do have to talk to MS Support, and where they can be helpful. Assuming you have Office Pro, after you explain the problem them should allow the activation to happen.