signing up for this forum and being blocked

I pressed the sign up pop up at the bottom of the screen, entered my email & password, then a very small window came up asking: "Enter the password for the user 'my name here' to continue." Above the password box it said: "enter your password to allow this." To be honest my first thought was that I had literally just finished signing in to apple, then I thought it must just be checking I was who I said I was, so I entered my password and pressed enter. the smaller box just shook and erased my password. I didn't type it in wrong, it was the right password, but if anyone can help I would be extremely grateful. Thank you in advance! Nat!