There isn't any PowerShot SX40 HS software available for Mac OS 10.15. There is a program built into your operating system you can use to import your photos. It is called Image Capture and the steps to use it are:
1) Click on the Finder icon in your dock (the square smiley face with a line down the middle).
2) Select Applications.
3) Image Capture will be listed in here, double click it to open it.
4) Once Image Capture is open connect the camera and computer with the USB cable. At that point Image Capture should see the camera and begin showing the photos.
5) Once the photos are loaded in you can select the folder you want to save to at the bottom of the window and you can import with the Import button in the bottom right of the window.
If that doesn't work with the USB there is one other thing that you can try. The steps will be the same, but on step 4 you would plug the memory card into a memory card reader.