When attempting to configure and edit a Dynamic Trend View Dashboard in CAPC (20.2.7), both the "Add Items" and "Add Groups" selection boxes are greyed out and so no items or groups can be selected:
DX NetOps CAPM 20.2.x and later
If you want to add items instead of groups, you now need to remove the group we add by default, when switching to Fixed (which is the current group context I believe)
To use different groups, click the Add/Remove Groups button to the right:
Once you remove the default Group, you should be able to add the Groups on your system.