failing to install sagemath on ubuntu 18.04 LTS
The PPA you mentioned does not have packages for 18.04 LTS.
In case of Ubuntu 18.04 LTS Bionic Beaver you do not need any PPA to install SageMath.
You can simply remove the PPA (if added before):
sudo apt-add-repository -r ppa:aims/sagemath
and then install sagemath from repository:
sudo apt-get install sagemath