Can't create itunesconnect sandbox tester

I had to use at least 1 upper case, 1 special char (like @), 1 punctuation mark and 1 lower case and be at least 8 digits long

I was getting simply 'This field is invalid' in the password when submitting - no guidance at all as to what was wrong - great UX Apple

I was just running into the same issue. I was able to finally get a test account through when i removed the +abcd from my address and just used another account I have.

So even though they say to use the +abcd thing it throws and "Unknown Error" when you try and use it.

I had the same error, but this wasn't my issue. In my case, I had created throwaway accounts that had 3 parts in the domain part of the email address - e.g. foo.bar.com. It turns out that won't work for iTunes, but two parts work fine - e.g. foo.com.

Thanks Apple for such a wonderfully opaque error message. Loved wasting a few hours on this.