How can a classical phase space be unquantizable?

Question

On page 2 of the paper "2 + 1 dimensional gravity as an exactly soluble system" Witten claims that:

Depending on its topology, a finite-dimensional
phase space might be unquantizable,

How a classical phase space might be unquantizable? Is it special to finite dimensional phase space or some infinite dimensional phase spaces also might be unquantizable?
What are sufficient and necessary conditions quantizability of a classical phase space?
Is not it a problem for quantum mechanics which some classical system have not quantum counterpart?

Qmechanic · Answer

Ref. 1 is likely referring to the integral quantization condition
$$frac{1}{2pihbar}omega~in~H^2(M,mathbb{Z})tag{1} $$
in geometric quantization of a symplectic manifold $(M,omega)$, aka. classical phase space, cf. Ref. 2.

The condition (1) is related to the Bohr-Sommerfeld quantization condition, cf. e.g. this Phys.SE post.

Note that the remainder of Ref. 1 uses covariant phase space quantization, cf. Ref. 3.

References:

E. Witten, 2+1D gravity as an exactly soluble system, Nucl. Phys. B311 (1988) 46.

N.M.J. Woodhouse, Geometric Quantization, 1992; section 8.3.

C. Crnkovic and E. Witten, Covariant description of canonical formalism in geometrical theories. Published in Three hundred years of gravitation (Eds. S. W. Hawking and W. Israel), (1987) 676.

How can a classical phase space be unquantizable?

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