'not very convincing' means 'not very believable'/'not very plausible'. A synonym for 'not convincing' is 'unconvincing'. If you look that up if may help you more in understanding. Saying something is 'not very convincing' is saying it is very unlikely someone will believe that thing is real/genuine. "The imitation bananas were not believable - they did not look like genuine bananas." "It is implausible that buyers will believe these imitation bananas are real." "These imitation bananas are unconvincing or hard to believe"

wow thank you for another usage of convincing you mean that convincing could mean ' wonderful' i got it right?

“Convincing” used like this is a fairly high level. We would often use it to describe the performance of an actor. “Tom Hanks was convincing in his role as a castaway.” “Saoirse Ronan does a very convincing American accent.”