TOPIC:

Although you may not be using the layer features, be sure that you have the correct floor level selected. You can determine which floor (or layer when used) by switching off each floor in turn to see when the dimension line also goes. This at least confirms which floor (or layer) should be current in order to select the dimension line.

Other possibilities:

Check that dimension lines are still selectable by looking in Settings - General menu and then the Selection tab. Check that Dimensions are still selectable.

Is this a refresh problem? Where dimension is being deleted but the screen does not update to show this, Press F5 to refresh screen

Lastly- try saving and reloading the project to see if this resolves the problem.

I cannot delete a dimension from a 2d plan, the numbers can be selected & rotated etc but not deleted, the line will not even select. There seems to be only one layer?