Not enough free space

[Foxchild]

Yes, you need to resize the filesystem to fill the space you want.
However, for you information, UT does not officially support using apt for installing stuffs. Normally, the root is read-only although in the pinephone this is not the case.
Also, root is usually limited to around 2GB only. Not sure if it's also limited on the pinephone though. And don't expect to run desktop apps on it by installing from the terminal. To do that officially, you need to use libertine but I think it's not working on the pinephone yet. It uses a container for installing things and use xmir for running x apps (it'll be transitioned to xwayland eventually).

Thank you for the heads-up!

Well, I solved the issue by installing gparted and running it via x-forwarding on my PC. I wasn't able to resize the partition with fdisk, resize2fs and/or parted... not sure why. Running gparted directly on the phone won't work either.
Just in case anyones interested though.