Você precisa instalar algumas dependências separadamente, uma por uma. No meu caso, estava jogando:
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
ubuntu-sdk : Depends: phablet-tools but it is not going to be installed
Depends: ubuntu-sdk-ide but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
Então eu tentei instalar o phablet-tools pela primeira vez:
sudo apt install phablet-tools
Ele lança:
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
phablet-tools : Depends: android-tools-adb (>= 4.2.2)
Depends: android-tools-fastboot (>= 4.2.2)
E: Unable to correct problems, you have held broken packages.
Então eu instalei o android-tools-adb e o android-tools-fastboot, um a um, usando:
sudo apt install android-tools-adb
sudo apt install android-tools-fastboot
Então eu tentei instalar o ubuntu-sdk novamente usando:
sudo apt install ubuntu-sdk
E funcionou.
Então, o ponto principal é que, você tem que instalar todas as dependências, uma por uma. Aplica-se a qualquer pacote que você queira instalar, não apenas para o ubuntu-sdk.