There are multiple ways to validate a site.
I would certainly pursue Steve’s recommendation first to ensure that your host is not blocking Bing, as that would be a problem you want to resolve.
Sometimes, when we copy and paste strings into text fields, we might also add an additional blank space character. So you may want to check the string to make sure you don’t have any spaces in front or behind the string.
If it still continues to give you a problem, you might want to try the alternate method, which is to downloaded the html file provided by Bing and uploaded it to your top level directory via SFTP (or FTP, whichever your host provides), then try and verify that way. AIOSEO gives you a text field to make it so that you don’t have to upload the file, but the alternative method is there if you want to use it.
Some people don’t have access to ftp to their server and/or don’t have experience with uploading via FTP, so using the text string is the preferred method. But if you have access via FTP, you should easily be able to upload the file and validate. You may also want to clear your browser cache too, just in case.
If it still doesn’t work, then Steve may be correct in the assumption that your host is blocking Bing for some reason.