Full Idea
The possible denial of the parallel lines postulate does not entail that Kant was wrong in considering Euclid's the only viable geometry. If the denial issued in a contradiction, then the postulate would be analytic, and Kant would be refuted.
Gist of Idea
Euclid's could be the only viable geometry, if rejection of the parallel line postulate doesn't lead to a contradiction
Source
comment on Immanuel Kant (Critique of Pure Reason [1781]) by José A. Benardete - Metaphysics: the logical approach Ch.18
Book Reference
Benardete,José A.: 'Metaphysics: The Logical Approach' [OUP 1989], p.133