My partner encountered the same problem and going by posts on Twitter, this happens quite often.
The IT team of the petition website advised by email to use a different browser and also to try using a password of 15 characters or less (but more than 7).
This did not solve the problem for my partner.
She succeeded eventually, using a password of 15 characters and restricting her choice of special characters to those provided by the website as 'examples' of special characters (_#@*+?!-$).
Her hunch (unconfirmed) was that those examples of special characters are actually the only special characters allowed for password creation on that website.
Good luck and many thanks for keeping at it!
So in summary, try using:
- 8 to 15 characters
- at least 1 uppercase letter
- at least 1 lowercase letter
- at least 1 number or 1 special character
- if using special characters, use one of these: _#@*+?!-$