I wonder if this topic has already been discussed before, but I got complains from my guests, they told the captcha was invalid [they’ve typed a correct captcha but it still told the same]

Use TrueType Font for Captcha and Append Random Numbers to Captcha Word are enabled, chmood set to 777.

Now, where is the mistake? could it be a bug? :/
or maybe they took a long time to comment? *not sure if there’s a time requirement lol, for sure I don’t know how to fix this*

There is a time limit.  I believe the secure code generated is valid for two hours from when the page with the comment form is first loaded by the user.  So, if they load the page and don’t reply for a while then they could run into the time limit.

is there any option how long the secure code generated is valid? coz i don’t think they didnt reply for a while to write something on my commentpage, but i will ask them, thx

The time is hard-coded into the backend code.  You would have to hack the scripts to change the value.