Because it looks like you are using the Online program this year. If you use the desktop software, you can use Deluxe to enter a K-1. However, when using the Online software, you need TurboTax Premier in order to enter a K-1.
Why is there any difference at all? Deluxe should be Deluxe, no matter the platform, right? If there are material differences - which this certainly is a material difference - then it should be called something else so as to avoid confusion. Now I'm sitting in the position of having paid for the online version that doesn't work for me.
Although you can enter your K-1 into the desktop software, the software does not guide you during the entries of a K-1 unless you have Premier. So technically you can do it in Deluxe desktop if you are comfortable with tax forms but Premier desktop will provide extra guidance that Deluxe doesn't. In the Online world, there actually 4 different versions of the software. The desktop versions all contain every form available in TT, but the step by step guidance and help is limited based on the version. <a rel="nofollow" target="_blank" href="https://ttlc.intuit.com/questions/2942039-what-s-the-difference-between-turbotax-online-and-the-turbotax-cd-download">https://ttlc.intuit.com/questions/2942039-what-s-the-difference-between-turbotax-online-and-the-turbotax-cd-download</a>
And if you want to switch back - <a rel="nofollow" target="_blank" href="https://ttlc.intuit.com/questions/1901476-how-do-i-switch-from-turbotax-online-to-the-turbotax-software">https://ttlc.intuit.com/questions/1901476-how-do-i-switch-from-turbotax-online-to-the-turbotax-software</a>