You need to use a shell to execute your command. Use shell=True when you define the Task.
Environment variable expansion must made by a shell. Use shell=True when you define the Task.
Shell is expensive if you want to run many tests. Use only when is needed.
Probably you are checking existence of the stdout or stderr of a Task. You must put save_stdout=true o save_stdout in the Task if you need those files.
atheist executes the .test files with the same Python version that it runs. It you use new Python features in your .test files but you run atheist with a old version the problem appears. To solve that always execute atheist with the newer Python version that you have.