'simpleSecurityObject' requires attribute 'userPassword'

Entry (cn=admin,dc=example,dc=com): object class 'simpleSecurityObject' requires attribute 'userPassword' slapadd: dn="cn=admin,dc=example,dc=com" (line=12): (65) object class 'simpleSecurityObject' requires attribute 'userPassword'

Why does it say that 'simpleSecurityObject' requires attribute 'userPassword'. What password it means? I have done everything exactly as it is said here till the point where it says add the new content sudo slapadd -l init.ldif. After doing that this 'userPassword' appeared.

Regards, Villem Alari