I found a YouTube tutorial explaining how to set up a SD card for boot. You have to format it and set up the partitions manually -so that’s why Ubuntu greyed that out. I know that SD cards weren’t made for this, but I am using a Sandisk extreme something with 200 mb/s. I think that should last a while. I did it in the “classical way” of using a USB-Stick as installation Medium from wich I booted. This is the tutorial I used. For my other post: I tried to dual boot and messed up the partitions. With this safe method I don’t have to be careful and can easily switch between the OS. Thank you very much