Multiplication in Polynomial Rings

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I am attempting to use substitute elements in a quotient ring into a formula and then multiply the results together, but I keep getting error messages. Here is my code:

R.<x> = QQ[x]

I = R.ideal(cyclotomic_polynomial(27)(x))

R27.<a> = R.quotient(I)

f(y) = (1-y)-(1-y)^2/2+(1-y)^3/3

f(a)*f(a^2)*f(a^4)*f(a^5)

The error message that I get is this:

no canonical coercion from Univariate Quotient Polynomial Ring in a over Rational Field with modulus x^18 + x^9 + 1 to Callable function ring with argument y.

Any suggestions on where I am going wrong and how to fix this?