<p>I am trying to slice a model that is half a mm less than max width, but not successful.</p>

<p><a href="https://i.stack.imgur.com/puSzw.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/puSzw.png" alt="enter image description here"></a></p>

<p>What am I missing? Is there some minimum value less than maximum allowed, or something?</p>

<p><strike><strong>Edit</strong>: after changing the width to 220 in machine settings, slicing works.</strike> This is a dangerous thing to do, as it <em>could</em> damage the printer.</p>

Take a look at this post:
https://community.ultimaker.com/topic/15588-cura-23-not-using-full-print-area/
. As the raft/skirt/brim will fall outside of the build volume, Cura is not able to slice it. Look at the the answer by @ahouben. He suggests that if you want to use the maximum build volume :
adhesion type = brim
brim line count = 0
travel avoid distance = 0
horizontal expansion = 0
support horizontal expansion = 0 (if support is enabled)
draft shield disabled
ooze shield disabled
infill wipe distance = 0﻿
Note that in most cases brim with brim line count=0 will get you most of the way there
Try this and see if it makes a difference.