There could be several reasons why you are unable to block an email address in Yahoo. One possible reason is that the email address you are trying to block is not a valid email address or is not associated with a Yahoo account.
Another reason could be that the email address you are trying to block is already on your blocked list. Yahoo allows you to block up to 500 email addresses or domains, and if you have already reached this limit, you may not be able to block any more email addresses.