There is two issue

1. Wrong option
2. Confusing []

A.1. GENERATING A SASL DATABASE

saslpasswd2 -c -p -f <SASL_database_name>.sasldb -u [<domain_name>] <user_name>

1

This command creates or updates the specified SASL database, and adds the specified user name to it. The command also prompts you for the user name’s password.

Not true, man for saslpasswd2: -p pipe mode – no prompt, password read on stdin

So remove please option -p from the example command.

2

And probably delete [] from -u [<domain_name>] .
This little confuse me what is expected input. (Like list of domains?)

So final state looks like:

saslpasswd2 -c -f <SASL_database_name>.sasldb -u <domain_name>
 <user_name>