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As far as I can tell, this error message has nothing to do with solve. (See below for that issue, though.)

sage: R = PolynomialRing(RR,['a','b','c','d'])
sage: S = matrix(R,2,R.gens())
sage: S
[a b]
[c d]
sage: a
---------------------------------------------------------------------------
NameError: name 'a' is not defined

If you do PolynomialRing? then you'll see

      Use the diamond brackets notation to make the variable ready for
      use after you define the ring:

So it looks like instead that this is just unsupported behavior that you are trying to do. I'm not sure exactly why it's not supported, but it's not.

sage: PolynomialRing(QQ, 'w')
Univariate Polynomial Ring in w over Rational Field
sage: w
---------------------------------------------------------------------------
NameError: name 'w' is not defined
sage:              sage: R.<w> = PolynomialRing(QQ)
sage: w
w

Now, we still run into problems even after we do the "right" thing.

sage: R.<a,b,c,d> = PolynomialRing(RR)
sage: R
Multivariate Polynomial Ring in a, b, c, d over Real Field with 53 bits of precision
sage: a
a
sage: S = matrix(R,2,R.gens())
sage: S
[a b]
[c d]
sage: solve([S[0][0]==S[0][1],S[1][0]==S[1][1]],[a,b])
---------------------------------------------------------------------------
TypeError: a is not a valid variable.

This is a workaround:

sage: solve([SR(S[0][0]==S[0][1]),SR(S[1][0]==S[1][1])],[SR(a),SR(b)])
[[a == r2, b == r1]]

But the code in the error above pretty clearly shows that we need symbolic variables. To make this code more concise for more complicated situations, you could do

sage: list1 = [S[0][0]==S[0][1],S[1][0]==S[1][1]]
sage: list2 = [a,b]                             
sage: solve( [ SR(w) for w in list1], [ SR(z) for z in list2])
[[a == r4, b == r3]]

and that preserves the type of a and friends...