Thin spanner provided is WRONG size for slim caster nut assembly.
Locking flange interferes with cross stabilizer bar preventing seated position locking pin to engage.

I called the SFA company at 888-3323-6419 and suggest everyone out there do the same. The guy on the phone seemed to blame it all on Mandarin and said "˜I'd fix it if I spoke Mandarin'. This might be amusing except many of our most enjoyed possessions are made in China (e.g. iPhone, LG, Samsung, etc.) which do NOT HAVE this problem. So, in my summation it is the design/instructions which are flawed rather than the assembler. Or, the Quality Assurance department.
In any event, call the number and register your complaint/problem as I did in the hopes that the message will get to the design team to get things corrected.
I used a Vise Grip to hold the body of the caster in order to tighten the locking nut/recessed connector and then filed down the flange so the seat assembly would rotate fully allowing the locking pin to engage.
Would I buy it again - maybe.
Would I spend a few dollars more to get a higher quality working product - probably.
Buyer Beware!

A few issues, many others have stated:

It's not black, it has Omega Lift written all over the vinyl in white and yellow lettering.

One wheel does not touch the ground, and its one of the ones that used while positioned as a stool.

Same white residue, appears like moisture transferred from the packing styrofoam to the vinyl.

Two of the plastic end-caps had to be notched to allow clearance for the bolts, seriously guys, come on.

It comes with one stamped sheet metal 13mm wrench. It should have come with one that fits the nut on the caster. I don't have a wrench thin enough to attach the 2 head board casters and ended up having to use channel-locks.

On the plus side, it does seem to be very sturdy and supported my large frame with no problem. I also have no problem folding/unfolding and moving it around in general.

Tools Used:

5mm Allen
13mm Box End
6" Adjustable
Channel-Locks

The creeper is not black. It has a black background but the entire unit (except the frame) is covered in white and bright yellow Omega logos. Also, the unit arrived with nuts that do not screw the whole way down on the bolts to the frame for the two front (head piece) casters, making it unusable. There's also a step missing in the instruction manual indicating how to assemble the three pieces of the unit. There are only instructions for the caster attachment and switching the unit from the reclining to sitting positions. Finally, while the frame does appear that it will support the rated weight capacity, the cushions are not as firm as I would have expected. As such, it likely will not be very comfortable for heavier individuals.

I reached out to the manufacturer. They indicated that they had a design change and that it has not yet flowed through to Amazon (which they ensured they would follow up on). They also said that they would send out new front casters and bolts to me. However, given that the person with whom I spoke seemed really confused about my problem, I'm unsure if what I will receive will even be correct.

Once assembled it is great. I had to chase the threads on the casters with a die. One caster was so bad I just had to tighten down on it even though it cross threaded. Instructions are not great either. Beyond those issues, it is very sturdy, rolls well and is easy to fold. It would be a bit heavy to carry frequently. I got it mainly for being a stool. One other minor complaint is that the seat doesn't lock down so when you pick it up in the folded position, the seat flips up.

Nice painted finish. The steel frame is strong and has no burrs or sharp edges. It has 1 locking pin to keep it in whatever position you put it in. To change the position you pull the locking pin out and once you start moving the head rest portion you can let go of the pin. Once the headrest is in it's lowest, or highest position the pin will automatically lock in place. 1 hand operation but don't over tighten the bolts at the pivot point. The wheels are nylon with steel bearings and roll and swivel as expected. All of the fasteners are large diameter with locking nuts. Cushions are covered in plastic and stapled om the backside. It is rated at 450lbs and I believe it would hold that with no problem The packaging was fair. The box was taped heavily on one end. Several of the fasteners were loose inside the box.