Something doesn’t sound correct. You can go here and click the launch binder badge below the text to launch a session with Julia as the kernel backing the notebook.
If you add a cell below, after you run the second code cell there in the demo notebook, with the following content, you should see the output as 2.6930971830599457 when you run the new cell:
s = s +2
s
This new cell uses the namespace of the notebook which defined s as 0.6930971830599458 in the previous cell.
If you aren’t seeing this in CoCalc, something is seriously different.
Is the code in a cell causing the kernel to fail and restart before you try your second cell?