can't use a variable containing string

(Sorry for my bad English, I'll do my best)
I entered this in interactive lua prompt :

local target = "test"
local file = fs.open(target, "r")

And an occur happen :

Expected string, string

I saw tutorials about string type, how to read a file, about fs . . .
I don't know what to do, please help . . .

That code would work fine in a program, but the Lua prompt localizes variables to just that line. They'll be nil when you enter the next line. Remove the local keyword while you're testing code in the Lua prompt.

thanks you so much :)/>