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The error is indeed cryptic (see trac ticket #24526). The problem comes from the fact that your expression x^2 + 1 is not a proper polynomial with coefficients in the base field but a symbolic expression. You should do

sage: K = GF(3)
sage: R.<t> = PolynomialRing(K)
sage: K.extension(t^2 + 1, 'a')
Finite Field in a of size 3^2

You can compare

sage: parent(x)
Symbolic Ring
sage: parent(t)
Univariate Polynomial Ring in t over Finite Field of size 3