Check out the following input screen:

1. On the left panel, click on Income > Passthrough K-1's > Partnership Info (1065 K-1) or S-Corp Info (1120S K-1);
2. Go to the State & Local tab;
3. Scroll down to Montana Adjustments {MT}; and
4. Enter the amount(s) under the MONTANA column.

It doesn't list MT in that area. I checked to make sure that state is marked and it is.

You may want to take a second look...

I contacted support since I couldn't find it and after an hour they realized that it wasn't supported in 2018 and that's why I couldn't find it, so I either have to do it using another method for preparing the tax return or just not use that deduction.

You should have stated 2018 in original post.

The logical assumption we volunteers make is that you are working on a current year return unless otherwise specified.

Yes, I realized that after the fact. I apologize for that.

This return has 12 states and has been a total time killer, to the detriment of my other clients, especially since I don't deal with most of these states. I'm used to calling customer support, but this year they are asking me where on the tax return the item should go. I previously used Lacerte, but customer support may be this way across the board.

Once again, I'm sorry, but at least I'll know how to do this for the 2019 return.

No problem. I know you're learning.

As Susan says, that's not a problem.  We're glad our input helped you save a couple minutes.  That's where this forum comes into play - sharing our collective experience to help one another.  Cheers!