Problem solved just warm record, but also to review the future review.
In practice, when Linux command, enter the command tree, but the display:
unable to locate package tree; then suggested that I could enter the download command: apt install tree;
But after input or not, what seems to be updated package, specifically prompted forgotten, and online information that can be entered apt install update;
But then input display: unable to locate package update;
This is a bit uncomfortable, and then check the information, to solve the following:
When the package is often an error message appears with apt-get update Unable to locate package update, especially on ubuntu server, the solution is:
First update apt-get, that is, enter: apt update, then it will run very smooth process while updating the command line ran very smoothly,
After the implementation, the problem is solved.
Then you can come back down to perform apt install tree, then the tree can be successfully executed commands.
Reference blog: