60-year-old Messina Denaro, who was arrested at a private cancer clinic in Palermo in January after 30 years on the run, had not appointed another lawyer, the presiding judge said.
Denaro had been due to appear via video link from the maximum-security prison in L'Aquila where he is being held.
Calogero Montante has subsequently been appointed as his lawyer.
The Trapani boss has been condemned to life in prison in absentia for his involvement in dozens of murders.
These include the 1992 bombings that killed anti-Mafia magistrates Giovanni Falcone and Paolo Borsellino, the killing of Giuseppe Di Matteo, the 12-year-old son of a mobster-turned-State witness who was strangled and dissolved in acid in 1996, and bombings at art and religious sites in Milan, Florence and Rome that killed 10 people and hurt 40 more in 1993.
ANSA