If you recently upgraded from Catalina to Big Sur, you will not be able to use itunes producer to publish your ibooks written with "Author." If you are not willing to wait for the engineers to come up with a solution, the only other way out is to down grade back down to Catalina and re-install ibooks. If you do this, don't use Time Machine to back up your files, because time machine is not compatible across OS versions. The best way is to copy and drop your files onto a DDS chip.
Apple is dropping it's support for for ibooks Author. If you write teaching materials and use the quiz widget, you will notice it can not be used in "Pages," the so-called replacement for ibooks Author. Knowing this, you will want to get back to Catalina. Your upgrading days are over if you want to continue to use ibooks Author.