Convert A4 Indesign brochure to A5, ready for print

Hi there, I purchased a brochure template from graphicriver.com. It is in A4 format. I need it in A5. I know how to convert it to A5 and how to 'print booklet.' However, when I try to print it booklet, it comes up with this error: "the active document uses multiple page sizes. print booklet works only with documents that use a consistent page size" Therefore, it will not allow me to print. However, I know that it doesn't use multiple page sizes because I can see the document. It also works perfectly when I print the original document in A4 instead of A5. I need someone to: 1. Help me make this A5 brochure 'printable' 2. Write out an instruction sheet on how to do it so if I need to do it again, I can repeat it following your instructions. I have a preferred timeline of 24 hours. Thanks, M.