That seal fits onto the bottom of the change over valve assembly, the reason you have no suction from the wand is due to this seal not being on.
The change over assy needs removing cleaning and the seal fitting back on the bottom. There is a couple of holes to align up, but in the past i have used a smear of uhu as well.
The seal comes off i find when the assy becomes dry and dirty, when i reassemble them i use a little silicone spray as the seal wipes over the lower hole.
The assembly is the part that moves left and right as you remove and refit the wand, you will see it swinging.
To remove is awkward, you have to remove the small torx screw on the middle of the teardrop, carefully and also when refitting this screw as it damages the plastic guide clip behind if you overtighten.
Once off, ease out the little flexi hose, you will think it's impossible to get out but it will.
clean and wash the whole hose and seal, renew if neccessary... or refit.
Quite a common occurance.
Sam has a couple of pics in his stripdown too.https://manchestervacs.co.uk/DysonForum/index.php/topic,1060.0.html
It's a 10 min job, for those with experience, 40min if you never done one before. You will require a small torx bit driver for the screw.