Occasionally people do have problems with the Captcha code.
The codes are always five characters, a mix of upper case letters and numerals.
If you find it difficult to read one code you should be able to request another.
If these hints do not help then you need to report the URL of the forum in question so Nabble can investigate further.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.