If you have auto-attach PDF turned on, Lacerte will attach a statement of aggregation to the return. If the auto-attach is not turned on, Lacerte will generate this e-file stopping diagnostic:

e-file: If two or more businesses are being aggregated for purpose of the Qualified Business Income (QBI) deduction, Schedule B - Aggregation of Business Operations must be attached to the efile. (ref. #51823)

In the upper right of Lacerte if you typed in the Search box 'Attach PDF' it will bring up this knowledge base article:

Attaching PDF Documents to an E-file Return

https://accountants-community.intuit.com/articles/1606113&src=lsccom

See the Note under the video. This tells you how to turn off auto-attach PDF, check to see if yours says yes to verify that it is turned on.