This is quite common in the software world but also a completely random OSX - Catalina bug which means that your system has not managed to download the Launcher file that you need to open the software.
Thankfully it is really easy to fix.
1) Move vanspace 3D App to apps folder.
2) Right click and click show Package Contents.
3) Open Mac OS Folder.
4) See executable file which Catalina has changed to a text file.
5) Open Terminal via Utilities.
6) Copy and paste the following into Terminal, you must leave a space after the x chmod +x
7) Drag the Text file from step 4 (vanspace 3D.txt) into the terminal window.
8) Press enter on terminal window.
9) The File will then change to a Unix executable.
10) Double click this file which is now an executable file (.exe) then follow the process as per the website tutorial.
PLEASE NOTE it is very easy to enter something incorrectly when following the steps above and there are two areas where it seems the most mistakes are made.
For Step 6 please type chmod +x exactly as you see it written including the plus sign AND a space at the end. Do not press enter, go directly to step 7
You can see how easy it is to complete the above steps here https://www.loom.com/share/bf4863f4c47d476593c7c42b68d2b551
Once completed you will then be told that the software is from an un-recognised developer .You then need to follow the onscreen prompts to create an exception and allow the software to Launch.