I got the travelers backpack and I right clicked on it and it put it on It when I go to my backpack slot it doesn't show the backpack and it wont let me access it in any way tried changing my key binds didn't work.

I also have this problem. My backpack is on my back but I cannot open it nor does it show in the slot. I can actually put ANOTHER back pack on :(

mod version

Same problem
Version: 7.1.8 - FML 40.1.73
Modpack:
WaterMineModpack-waterfall-5.2.0.zip

SCREENSHOT (no any wearer)

SCREENSHOT (force wearing with shift+click and after wearer try to press B or check curious)

SCREENSHOT USING OTHER BACKPACK FORCING WEARING

TO GET BACK THE BACKPACK
Just die, corail never wearer backpack again

is it singleplayer?

#476
See this, there's solution. If it doesn't fix your issue, open again.