Reference:
An important thing to remember when configuring virtual-link is we need to configure the OSPF router ID and NOT the IP address of the ABR.
Therefore in this question we have to use the command "area 234 virtual-link 0.0.0.44" on R2 and "area 234 virtual-link 0.0.0.22" on R4.

The "summary-address" is only used to create aggregate addresses for OSPF at an autonomous system boundary. It means this command should only be used on the ASBR when you are trying to summarize externally redistributed routes from another protocol domain or you have a NSSA area. But a requirement to create a summarized route is:
"The ASBR compares the summary route's range of addresses with all routes redistributed into OSPF on that ASBR to find any subordinate subnets (subnets that sit inside the summary route range). If at least one subordinate subnet exists, the ASBR advertises the summary route." But in this case we found no prefix that belongs to 10.0.0.0/8. Therefore a summarized route for this subnet could not be created.
Note:
+ If a prefix of this subnet exists in the routing table then after the summarization is performed, we will see such an entry:
Router# show ip route
- output omitted -
0 10.0.0.0/8 is a summary via null0

In this IP SLA tracking, we don't need a IP SLA Responder so the command "ip sla responder" on R6 is not necessary.
We also notice that the ACL is blocking ICMP packets on both interfaces E0/0 & E0/1 of R6 so we need to allow ICMP from source 10.10.10.1 to destination 10.66.66.66.