Installing Ubuntu Server: Selecting boot partition failure

I’m trying to install Ubuntu Server on my laptop where Windows is installed.

Via Gparted I’ve downsized Windows partition and created 3 partitions for boot, fs root, and home.
But when I try to setup partitions during installation wizard it keeps asking to select a boot disk despite I’ve chosen one already.

I’ve tried to select a Windows boot partition (I’ve seen somewhere that if boot partition is to far away, like > 100Gb then it might not boot), still same situation.

If I try to delete a partition then I got "Can’t delete a single partition from a device that already has partitions"

Creating new partitions even if there is some space also not available in the menu of the disk.

Any thoughts or suggestion?

Thank you!

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• Laptop: ThinkPad W520
• UEFI/Legacy loading
• Legacy first
• Windows is installed and I don’t want to delete it for now
• Ubuntu Desktop installer don’t offer to install Ubuntu together with Windows(like cant see Windows). Windows partitions can be mounted in live Ubuntu
• 2Mb grub partition added during installing Ubuntu Desktop but cant see it in loading options list of the system. Can’t load Ubuntu Desktop that I’ve just installed. What else I’m missing?

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UPD. I’ve added 2Mb grub partition but cant see it in loading options list of the system