Windows 10 volume reduction is not possible?

I would like to install Linux on my Surface Laptop (1st Gen) with 128GB storage in addition to Windows. For Linux 25gb is enough for me. There are currently about 45GB free on the laptop. The partition manager says, however, that I can only shrink max. 9gb. Defragmenting did nothing.

Does anyone have any idea what I can do there?

In principle, you can also do this with Windows tools, but it is much easier with special tools, e.g. With MiniTool Partion Wizard:

https://www.computerbild.de/...46547.html

After you've redistributed your hard drive with it, click Apply in the top left. Only then will the changes actually be made. Since drive c is your active partition, your computer must first reboot and then make the intended partition changes before Windows. Windows should then automatically boot properly again.

Before you start repartitioning, however, you should completely back up your hard drive to an external data medium and use a suitable recovery medium, e.g. Stick, feature. Because safe is safe…