NixOS can't remove Linux Mint Debian Edition Partition

I’m trying to install NixOS on a laptop that was using Linux Mint Debian Edition.

First I tried installing Nix next to LMDE, but that didn’t work, as the installer told me it couldn’t resize the LMDE partition. I didn’t have anything on the laptop so I’m now trying to wipe the board and do a complete install of Nix, but I’m getting the same error.

Here’s the log:

I also tried earlier to resize the partition using gParted, but it couldn’t access the partition somehow.

The LMDE boot partition seems to have been deleted because the laptop won’t boot at all anymore if there’s no LiveUSB plugged in.

How can I remove the LMDE partition?