I cant reinstall my rear cassette!

[Ajax Bay]

Yes, the seal has not been pushed back in (should be deeper and pushed over the spacer on the axle (just push with a blunt headed 'thing')) after the OP had serviced the bearings.
You alluded to that suspicion two days ago!
Lock nut still seems too far out (beyond freehub edge) to me, mind. Should be about 3mm max.
I'd like to see the other end of the axle.
I assume this is an 11sp freehub and the 9sp cassette needs a 1.85mm spacer.

 

[EckyH]

You alluded to that suspicion two days ago!

The OP gave us the hint in the first post:

the problem is getting the lock nut into the locking wheel once the cassette is back on. I can only finger tighten the locking wheel and the lock not has no room to engage the lock nut once everything is in place. The spacer is on before the cassette so I know there's no issues with the cassette itself.

The photograph in post #10 was sort of a validation.

Lock nut still seems too far out (beyond freehub edge) to me, mind. Should be about 3mm max.
I'd like to see the other end of the axle.
I assume this is an 11sp freehub and the 9sp cassette needs a 1.85mm spacer.

That's why I asked for the hub model: to get the facts instead of suspicions, assumptions or speculations.
As Dr. Kate Wilding used to say: "Amateurs speculate. And have to apologise afterwards."

E.

 

[Ajax Bay]

Arrow points to the seal that needs to to be pushed in.

 

[Gunk]

Usually it’s the smallest cog that causes this problem, you’ll need to rotate it until it clicks home and then reinstall it

 

[T4tomo]

I hope not as this bike is only a month old! Bought brand new.

WTF are you re greasing wheel bearings if it's only a month old?
Or is this another one of those wind up threads