Symbol's value as variable is void: bibretrieve-backends

[Gianmarco Brocchi]

I get this error when running M-x bibretrieve and entering Author and Title.
When I run C-u M-x bibretrieve, after entering Author and Title I get:

Symbol's value as variable is void: bibretrieve-installed-backends

I am using GNU Emacs 27.1 and AucTeX: 13.1.4, and I have installed bibretrieve from MELPA.