This is really odd. It seems like it should be pretty simple.

When editing the backend entry, I expand the Access control lists and actions section.

Then I create an ACL named certNotValidACL
with expression type SSL Client certificate valid.

Then I create an Action of http-request deny for condition certNotValidACL.

After saving and clicking Apply Changes I get the following error:

Errors found while starting haproxy
[NOTICE] (99213) : haproxy version is 2.8.3-86e043a
[NOTICE] (99213) : path to executable is /usr/local/sbin/haproxy
[ALERT] (99213) : config : parsing [/var/etc/haproxy_test/haproxy.cfg:52] : error detected while parsing an 'http-request deny' condition : no such ACL : 'certNotValidAcl'.
[ALERT] (99213) : config : Error(s) found in configuration file : /var/etc/haproxy_test/haproxy.cfg 

And sure enough, the test haproxy.cfg file is lacking the ACL entry.

Thanks in advance for any help on this!