There are a couple options. One make sure you're going to the main tagset and hitting Edit at the top. If you are and still getting that error, you can try clearing your browser cache.

If you're still getting those errors, it's a problem on ao3's end. You can use a second account to nominate or you can post your nominations either here or e-mailing them to hurtcomfortexmod@gmail.com and we'll add them in for you.