The Form 3514 requests a business code, business license number and SEIN. If you don't have a SEIN or business license number, then you should be able to leave those lines blank, per the form instructions.
These are both state or locality issued numbers. Sometimes the software is just verifying the information in the error/notifications. The schedule K-1 doesn't report this information, so you wouldn't have it unless it was included in supporting details.
You will need to enter the business code (for the type of activity), per the instructions. You can search for a 6 digit business code that fits the business activity at the NAICS search code listing here: https://www.naics.com/naics-search-results/
If you don't know what the business activity is then you can use code 999999.
