First of all, I think that non-native speakers are perfectly eligible to follow certificate courses. In my opinion, they have as much chance of succeeding as native speakers do, as long as they have a level that allows them to study and write essays in English. As for what you say about making mistakes, many native speakers also make mistakes. This often happens with language we know well and can be considered ‘a slip of the tongue’; it doesn’t mean you or I don’t know the language point involved.
I suggest you contact the course provider and ask them what their criteria are for joining the course. I don’t know the course you mention nor whether it is accredited. I suggest you contact the school and ask them if they are accredited and whether there is any external body that validates the course.
One thing to remember is that on-line courses present many advantages for trainees but one disadvantage is that trainees don’t always get the opportunity to teach with an observer present. Observation is an important aspect of teacher training because an experienced teacher gives you feedback on what you are doing.
You can also take a look at the TEFL Course Database on this website by following the link below: