For the purpose of adding ibus-pinyin in ubuntu, we are required to make a few modification to the system.

Firstly, we may install the corresponding package

Bash
sudo apt-get install ibus-pinyin -y

Secondly, restart the ibus

Bash
ibus restart # no sudo required

And then, we can add it in our system settings: Settings -> Region & Language -> Input Sources -> [+].

We can find an option Chinese (China). Click this button, and then we will find a list like Chinese(Cangji3) and so on. Choose Chinese(Pinyin).

We should able to switch the input methods using command + space.

You may also use this link as reference. ibus-sunpinyin could also been a good alternative solution