Steps to reproduce:
Create a form with 3 numeric fields (NumericOne,NumericTwo and NumericThree)
Create the following rule: If NumericOne is not empty and If NumericTwo Is not empty Calculate NumericThree
Click in the three dots on Calculator and select Sum.
Actual Behaviour: When you select Actions (triple dot) and then Sum, you cannot select any fields (the Add Field button is greyed out)
Expected Behaviour: When you select Actions (triple dot) and then Sum, you should be able to select fields and use the Sum Action feature.
I recorded a video to show this situation, here is the link: https://www.loom.com/share/698865dc4f40486596fe725ba3e73ad3
Master @ 449d3d0fc4a72aa491432bf253cfd76f33601cbc
7.3.x @ df892f7f367e483ce903de507522c20e57922789
7.2.x @ 1c8b5ae7550c2b98bcf08e9a34f0a1c165e2b24a