Participant can´t enter the study

I have serveral participants who can´t enter my study (after they have received an email) although there are enough places left. I get messages like this (see below). Any idea what the problem might be?

Thanks for your help.

I got a mail with an invitation to this study twice but I can’t take part in it because the notification says ‘You are ineligible to take part in this study.’ I’m not sure if there is some kind of technical problem.

Hey Simone, welcome to the community! :tada:

I’m really sorry that you’ve been having issues.

When did you receive this message, and what is the name of the study experiencing problems?