An question about VarD

[chandra]

 I am using xTras to do some calculation, but I face a problem,

 I do not know how to fix it. I read the paper “eTras”, a field-theory inspired xAct package for Mathematica, 

and find the following Mathematica code:

When I try to repeat the code in my computer, there is an error,

I do not tell Mathematica any information about phi[] because I do not know how to do it. I think this must be the reason for the error. Could someone tell me how to fix it? Thanks in advance.