Today while opening the garage door with chamberlain system, the door bracket came off, inside of the holes looks like fibre. What would be the best way to fix the stripped holes

 

Can you use a larger sheet metal screw?? Those look like they might be #12 or #14 You could try the next size up, but stay away from Tek screws....just get the pointed sheet metal screws like the two short ones in your hand. Take them to the hardware store with you and compare sizes. Be careful to tighten them, but not strip the threads when you install. Possibly one or two of those were stripped from the original installation.
If that doesn't work, then you might have to improvise with anchors, or a mending plate that covers a larger area, and then the bracket on the plate.
Looks like kind of a shoddy way of installing the bracket that has so much stress....I don't know if that is industry standard or not. Just my opinion though.

 

Welcome to the forums.

Not sure what the vertical bracket is there for. Doesn't help support the opener bracket.
You could try larger screws but they may not hold well either.

My recommendation would be to replace that bracket with a larger reinforcement bracket.
Reinforcement bracket

 

you need an opener bracket, there is no re-enforcement backing plate for the screws to grab into under the skin at that location.. see here
And looking at your picture again it appears the re-enforcing strut on the top of the door is too low on the section to work with an opener bracket it will probably have to be moved up. It also has no screws in it where your bracket needs to be secured.