Debentures cannot be issued by a LLP. Definition of debentures as per section 2 (30) of the Companies Act, 2013 gives power only to company for the issuance of the same.
Definition is reproduced below for your reference,
"debenture" includes debenture stock, bonds or any other instrument of a company evidencing a debt, whether constituting a charge on the assets of the company or not;
(a) the instruments referred to in Chapter III-D of the Reserve Bank of India Act, 1934; and
(b) such other instrument, as may be prescribed by the Central Government in consultation with the Reserve Bank of India, issued by a company,
shall not be treated as debenture;]