Explanation/Reference:
Explanation
First we should understand clearly about the line
Dead R 120 C 10, Hello R 30 C 30
The R here means Received and C means Configured. In other words, Dead R is the Dead Timer Received from the neighbor and the Dead C is the Dead Timer of the local router.
Therefore in this case Dead R 120 C 10 means the Death Timer of the neighbor is 120 seconds while the local Dead Timer is 10 seconds, which causes a mismatch. Also we can learn that the local OSPF dead timer is set to 10 seconds.
For your information, by default, OSPF uses a 10-second hello timer and 40-second hold timer on broadcast and point-to-point links, and a 30-second hello timer and 120-second hold timer for all other network types.