I created a performance and want to assign two volume controls to the super knob but have a problem with the second.
In Performance Home I select the volume slider of a part in the touch screen, press Control Assign and rotate the Super Knob.

When doing that I get the message "Assignable knob full".

We don’t have enough information to tell exactly what’s happening. But when it is said you are assigning something to the Super Knob what you are actually doing is linking a Part Assign Knob to a Common Assign Knob.

The Common Assign Knobs are the ones linked to the Super Knob. Your error message is probably due to the fact that you either already have 16 Destinations for either the Part or you have 16 Destinations already occupied at the Common Settings level of the architecture. Or simply all 8 Part Assign Knobs are already occupied — in which case, you cannot automatically have the [CONTROL ASSIGN] function make an additional assignment. In order to have more than one parameter assigned to a Part Knob, you must manually review all assignments to the specific Knob you wish to assign another parameter.

Here’s how to know:
From the HOME screen
Touch Part 1
Press [EDIT]
Touch “Mod/Control” > “Control Assign”
Here you can review the Control Assignments… 4 Destinations per PAGE, 4 PAGES
If you already have 16 Destinations already made for this Part, your “Assignable Knobs are Full” for this Part.
Each Part has 8 Knobs and 16 Source/Destination Control Sets.
Review each Part Knob assignments.
Check each specifically.

Check each of your Parts.
If you do not have a Source/Destination available you’ll find it here.
If not, get back to us with a bit more information…

Review your assignments, likely you are not even using some of them — since you have a MODX (which is Ribbon-less) — you’ll find the Ribbon as a Source in some of the Assignments… you can eliminate Control Assignments you are not using by highlighting the Destination and then tapping “Delete” in the lower right corner.