Is the Hadith "It was said: 'O Messenger of Allah, can a believer be a miser?' He said: 'Yes.' They said: 'Can a believer be a coward?' He said: 'Yes.' They said: 'Can a believer be a liar?' He said: 'No'" authentic?

The hadith you asked about was narrated by Malik in Al-Muwatta, and Al-Sakhawi and Al-Ajlouni described it as mursal (lacking a Companion in the chain) or mu'dal (missing two or more consecutive narrators), and Al-Albani said about it: "Mursal, da'if (weak)."