TRT is already using table calculation and you can not use Runing_sum.
Then my approach is using this worksheet as new datasource.
Edit new datasource
add filter to filter out "Null" from this new datasource.
Now as second datasource you get TRT as "simple value" not table calculation.
RUNNING_SUM(SUM([Turn Around Time]))
TRT_2_rep_9.2.twbx 67.4 KB