安裝軟體出現 E: Unable to locate package

當使用 apt 安裝軟體時出現下面的訊息:
$ sudo apt-get install <package>
Reading package lists... Done
Building dependency tree      
Reading state information... Done
E: Unable to locate package <package>

開啟 Software & Updates，選Ubuntu Software分頁，勾選Download from the Internet選項:

執行 sudo apt update

再安裝所需的軟體:
sudo apt install <package>