Setting up custom variables in gitlab pipeline

Background

Having a new pipeline needed to setup, I try to put some frequently use variables e.g. AWS login ID to the custom variable convenient for my pipeline usage, but somehow it fails to read the variable 

Reason

Settings problem, I configure the variable as a protected variable, which only allows protected branches or tags to use only, as the target branch is not configured, the custom variable is not recognized

Solutions

We need to configure our target branch as a protected branch in Settings > Repository > Protected Tags, in that section we can input regular expression for the tags identified as "protected" branch, after that, the custom variable is recognized successfully