If you have checked if they are allowed to take Unreimbursed Partnership Expenses, you will first need to manually calculate the allowable office expense (use the worksheet in Publication 587 or use Form 8829 as a worksheet).

https://www.irs.gov/pub/irs-pdf/p587.pdf#page=21

Then on the K-1 worksheet, scroll down to line 20 Z and click the Quickzoom for Unreimbursed Partnership Expenses (it doesn't matter where you enter it on that worksheet).

thank you!