What printer do you have?

It may be that it is incapable of producing 51 micron (0.051 mm) layers and Chitubox knows this. My plain ol' original Mars cannot do it as it has 1.25 micron (0.00125  mm) Z-axis resolution (I.e. it can only move in 1.25 micron (1/20000") increments)--51 microns would be 40.8 Z-axis "steps", obviously it cannot do 8/10ths of a step, 50 microns is 40 "steps",  41 "steps" would be 51.25 microns. However Chitubox realizes the absurdity of "thinking" in 0.05 micron increments and only allows 1 micron resolution values to be entered.

40 microns (0.040 mm) is be an even number of Z-axis steps (32), something the machine can do...

For reference the average, reasonably accurate digital capliper has 10 micron resolution.

Also, what would be the point of 51 micron vs. 50 micron layers?

This subject got me wondering about the Z-axis resolution of common SLA printers--did some research and this is what I found.

Turns out that Elegoo printers are the clear winners at 1.25 microns, it's the only one (save the original Photon S) that could even come close to using 51 micron layers. BTW, Chitubox v1.9.1 does accept and persist a 52 micron layer thickness setting for my plain ol' Mars printer.

Again to establish a reference point, ±0.001 mm (±1 micron) tolerance is considered "high-precision" machining by contemporary standards. Starrett's $700 T221Xl high-precision micrometer has ±0.00005" (±1.27 micron) accuracy...

The reason why I want 0.051 is because this work for me: https://www.youtube.com/watch?v=Qs2Rb0ExnIM
When I used lychee... But I like the auto support better in Chitubox, so I want to switch. 

That is all quite interesting, however is your printer capable of printing 0.051 (51 micron) layers?
The fellow in the video has an Anycubic Mono, which though having 0.051 mm (51 micron) X and Y "pixels", has 0.01 mm (10 micron) Z-axis resolution. I.e. it is incapable of printing 0.051 mm (51 micron) layers--a fact the producer of the video has overlooked. Most likely Lychee has ignored this illigitimate request as Chitubox seems to do.

What printer do you have?