To what extent is the hadith "Why should I not be enraptured while the space between the fingernail and fingertip of one of you is filled?" authentic, and can it be used as a proof?

The hadith is weak and cannot be used as proof. Al-Bayhaqi narrated it as mursal (with a missing companion in the chain), and Al-Bazzar and Al-Tabarani narrated it as mawsool (with a connected chain). It includes Al-Dahhak ibn Zayd, about whom Ibn Hibban said, "It is not permissible to use him as proof." Al-Albani judged it as "munkar" (denounced).