mathics -c 'xx="a string"; StringTake[xx[0]]'

In[1]:= xx="a string"
Out[1]= "a string"

In[2]:= xx[0] //FullForm
Out[2]//FullForm= "a string"[0]

StringTake::argr: StringTake called with 1 argument; 2 arguments are expected.
StringTake[a string[0]]
```

Notice that if instead of a string, the head was an integer, the result was already right:
```
(Mathics3) mauricio@mauricio-T15thinkpad:~/Projects/mathics-core$ mathics -c 'xx=7; StringTake[xx[0]]'
StringTake::argr: StringTake called with 1 argument; 2 arguments are expected.
StringTake[7[0]]
```