As the error it's saying, the disk where you want to install the application is full. You need to delete some files and then try again. I've downloaded the latest version of SoapUI (Open Source one) and I've installed it on my hard disk, but I do already have 200+ GB free on the system drive.
If you can't delete files, simply point the installer to another folder on another partition. The total installation size as shown in the installer is 254MB, therefore you should make sure that there is enough free space on the target location before starting the installation procedure.