First of all, trap here is a technical soccer term: it's the action of stopping the ball with your foot, so you can more easily aim your next kick. You can see a demonstration in this video.
This passage is using a common idiom in English: if you want to say that someone is really bad at a certain task, you suggest that even if the task were changed to make it extremely easy, they still couldn't do it. Some examples:
Someone who is bad at shooting: He couldn't hit the broad side of a barn. (A barn is a large building and should be really easy to hit - someone who can't even do that must be really bad.)
Someone who's not good at convincing people to do things: He couldn't sell a bucket of water to a bloke [man] in burning trousers. (Someone whose trousers were on fire would be very eager to buy a bucket of water, but this person couldn't even make that sale.)
Someone who's stupid: He couldn't pour water out of a boot if you told him the instructions were on the bottom. Pouring water out of a boot is really easy, you just turn it over. If someone were really stupid, they might need instructions. If you told them the instructions were on the bottom, they would hopefully turn the boot over to read the instructions - when they turn it over, the water would pour out. If they still couldn't figure it out, they must be really, really stupid.
In this example, trapping a soccer ball is not that difficult. If you replaced the soccer ball with a bag of cement, which is very heavy and doesn't roll, bounce, or move much at all, trapping it would be extremely easy. So someone that couldn't even trap a bag of cement would be really bad at that skill.
These are all examples of humorous exaggeration or hyperbole.
The overall meaning is if he is really bad at trapping the ball, he isn't going to get goals.