I signed a sales og agreement specifying that the sale would be reiterated by acte authentique before November 2, 2019, i.e. 3 months after the signing of the sales of agreement. [...].

[...] The date has passed, the acte authentique has still not been signed and the seller is refusing to sign. What can I do?

This depends on the terms of your sales of agreement, but in principle, failure to meet the deadline does not result in the sale lapsing ("disappearing"). It merely allows you to give formal notice to the defaulting vendor to sign the deed, and if he fails to react, to refer the matter to the judge to request judicial completion of the sale or its resolution.