Instead of using the built-in tool I downloaded a partition master:
https://www.easeus.com/partition-manager/?ad&gclid=Cj0KCQiAw5_fBRCSARIsAGodhk8rpotKYGg3okbGx4EzOxtZXqJxKB9Ry8rjDBdFrHW7Ibo7tQ9m294aAoFSEALw_wcB
and used that to create a new partition. Think there's others which can be used...
I am trying to install Linux alongside Windows 10, but the installation stops, because it seems it's not possible to create a partition for Linux. There's about 100 GB free space on the harddisk to create the partition. I have tried to install both Linux Mint and Lubuntu.