Axle washer, not cassette washer

I bought a Zwift hub in February 2023. Today, I tried to install the Zwift cog on my hub, but I didn’t have a washer when I removed the gears. Is this a problem?

Not in the slightest as you’ll be aligning your derailleur up with the cog.

Ok, but what is the role of the washer ?

The spacer is there to fill a space on the freehub if you are using an 8/9/10 speed cassette on your bike. You are removing the cassette and replacing it with a cog which is designed to fit perfectly.
Therefore there is no gap to fill.

Do you mean this?

check it isn’t on the inside of the freehub. If there was grease on it it may have come off with the freehub body.

Yes I mean this

yeah, you’ll need that.

if is wasn’t there the freehub might have been wobbly so it should be somewhere. so see if it is stuck inside the freehub body.