Unless you're doing driver development or kernel development in an isolated lab without the risk of outside breakins or compromization, jumping in as root directly to the Linux OS is considered insecure.  So this post will not describe how to do that.  However we will describe how to enable root sudo to root via an unprevilidged user:

su: pam_succeed_if(su-l:auth): requirement "uid >= 1000" not met by user "root"

To get started, if you find this line in /etc/pam.d/ remove the uid >= 1000 part to allow for sudo to root from an unprevilidged ID:

auth        requisite     pam_succeed_if.so uid >= 1000 quiet_success

check:

auth        requisite     pam_succeed_if.so quiet_success

Cheers,
TK