The error should be looking as below:
You have an unresolved error when starting csf:
Error: Error processing command for line [339] (6 times): [Another app is currently holding the xtables lock. Perhaps you want to use the -w option?], at line 339 in /usr/sbin/csf
You need to restart csf successfully to remove this warning, or delete /etc/csf/csf.error
CSF recommends to set WAITLOCK to “1”
cat /etc/csf/csf.conf |grep WAITLOCK
# csf uses a last ditch timeout, WAITLOCK_TIMEOUT in seconds, that will trigger
WAITLOCK = “0”
Change it to “1”
WAITLOCK = “1”
remove /etc/csf/csf.error
rm -fv /etc/csf/csf.error
Start csf
csf -e
Source: https://support.cpanel.net/hc/en-us/articles/1500006499362-CSF-service-not-starting-Error-Another-app-is-currently-holding-the-xtables-lock
Source: https://support.configserver.com/en/knowledgebase/article/csf-will-not-start-and-i-am-getting-errors-in-varlogmessage