NO backtics in awk, please! And, using nested backtics like in your mv command needs escaping them. That's why backtics are deprecated and users are encouraged to use the $(...) form of command substitution.
In order to get the output of a shell command into an awk variable, you need to use command, pipe, and getline like in
BTW - the way you use the ls -ltr construct is far from optimal and would not fit mv usage, as both invocations may yield multiple files that mv does not handle that way. What's the reason to use it the way you do? Maybe there's other, improved ways to achieve the desired result.