Hi Koen,
You’ve previously installed the two necessary toolboxes, right?
Widgets Toolbox - Compatibility Support - File Exchange - MATLAB Central (mathworks.com)
GUI Layout Toolbox - File Exchange - MATLAB Central (mathworks.com)
Note that one of them says “compatibility support”, which is the suitable version for v2020a and earlier - make sure that you install that one and not the one for 2020b and later. MATLAB have (of course) overhauled their UI-building architecture in 2020b, and with that came a new Widgets Toolbox release that isn’t backwards-compatible. We’re still figuring out what to do about all of this.