Welcome to the Make community!
You cannot access variables in an Aggregator loop (gray shaded background), for modules outside of it.
If you want to do that, you have to use a Set Variable module inside to store the data.
Then, after the aggregator, use a Get Variable module to get the variable that you saved.
Alternatively, you can use an Array Aggregator module after the Clickup “List” module first, then Iterate the array.
This way, the Slack module can access the values in the Array Aggregator.
Hope this helps! Let me know if there are any further questions or issues.
— @samliew
P.S.: Did you know, the concepts of about 70% of questions asked on this forum are already covered in the Make Academy. Investing some effort into it will save you lots of time and frustration using Make later!