Question

This sentence may contain errors

Answer 1

Let's reconstruct this sentence. 

"Everyone I know complain" doesn't make much sense. 
Popping an "s" on the end of "complain" might do the trick, lets try!
"Everyone I know complains" ah, there we go. Much smoother.


Next section!

"Complains because these days backpacks weigh a ton."

Though this may sound correct, it is still quite a bunch. Let's fix that.

Rearranging your words might work.
Maybe in the section that says "Complains because these days" we can change that to "Complains these days because" Much smoother, and easier to read. 

We have so far is:  "Everyone I know complains these days because" 

Yay, let's continue.

The remaining of the sentence is fine so we can put it on the end of our freshly constructed sentence.
Our final sentence should be:
"Everyone I know complains these days because backpacks weigh a ton."