Azure DevOps provides a number of predefined system variables out of the box on hosted build agents.
While these have officially always been read-only, informally you’ve previously been able to change the value of a predefined system variable by overwriting it in a pipeline task.
However, as of the January 28 2020 DevOps release this is no longer the case. The DevOps team have improved the security of system variables, and as such now actively prohibit system variables from being written to.
The variables include items such as the DefaultWorkingDirectory, JobName and TeamProject. These are all variables that you can otherwise use and refer to in your DevOps pipeline tasks.
This means you’ll need to now create your own task or pipeline variables where possible. If you can’t modify the task or pipeline variables, speak to someone with admin rights who can.