how to run fraction elenment in Multivariate Polynomial Ring in over Finite Field ring

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Let's take this one step at a time. If you put lots of commands on one line then it is hard to see which one gives the error. The was to define GF(27) is

sage: k=GF(27, 'a')
sage: a=k.gen()

or more concisely

sage: k.<a>=GF(27)

Since it is not a prime field, Sage needs to know what name to give its generator, and also you need to define a variable whose value is that generator. Now

sage: F=Frac(k['xy'])
sage: F
Fraction Field of Univariate Polynomial Ring in xy over Finite Field in a of size 3^3

shows that you have defined a function field in one variable called 'xy', not in two variables. I think you meant this:

sage: F=Frac(k['x','y'])
sage: F
Fraction Field of Multivariate Polynomial Ring in x, y over Finite Field in a of size 3^3

Now for the rest of your input to work, you still need to define variables called x and y ( so far you have only told Sage to use 'x' and 'y' to output elements of F):

sage: x,y = F.gens()

Now the rest works ok -- but x/y is an element of F, there is no need to write F(x/y).

Please format your question, it is almost impossible to guess what your question is.

to me, it seems that you have defined the fraction field but have not defined the "variables". You should do

F.<x,y>=Frac(k['x,y'])

or

F=Frac(k['x,y']) x,y=F.gens()