There are a lot of such tutorials on the Internet. The basic pit is that fcitx is not installed with language setting. No fcitx is set. Delete the directory of sogou and restart fcitx.
Install Chinese in the language setting and restart it!
Here is a basic tutorial I found
First go to Sogou official website to download the Sogou input method sogoupinyin_2.1.0.0086_amd64.deb under linux. After downloading it is not enough, go to the software store to install fcitx, select the first one, and then open the settings, language support, in the Keyboard Select fcitx in the input method system, and then restart the ubuntu system