So it tries to fill and then aborts with a F51 or other code?

Does it turn at all, basket, agitator dome or anything? If it doesn't turn at all you've misaligned something. The kit replaces the seal, top bearing, spacer, washer, bottom bearing,shaft and nut? It also requires a bearing press to align and seat the two bearings. If all is normal the shaft will spin freely in the bearings.

The same stator and rotor drive the shaft whether it's in agitation or spin. Nothing changes under the machine between spin and agitate. It's just how the PCB powers the stator. Your tub floats up on that shaft to agitate and settles back down to spin. If it cannot float it will not agitate.
Reach into it when the machine is empty and grab the basket on opposite sides, then lift. If it doesn't slide upward freely on the drive shaft about 3/4" it can't disengage from the basket drive part of the shaft and will not agitate.

If the basket moves up and down but doesn't turn, lean it back and see if the rotor turns freely from beneath. If it doesn't, remove the rotor itself, inspect the rotor and stator for damage or a screw of something stuck to the rotor magnets, and see if the shaft turns by hand.

This is the kit...
Whirlpool W10435302 Bearing and Tub Seal Kit - AppliancePartsPros.com
and this is the tool to align it all
Whirlpool W10447783 Bearing Installation Tool - AppliancePartsPros.com

I've never tried it without the tool, but it would not be fun and I would have little confidence in it. It's about a 60 minute job with the tool and decent luck getting it apart. I've done more than I can count. 2 last week alone.
Good Luck,
Drew