There are several ways to create tuplets (Speedy Entry Tool, Simple Entry Tool, Tuplet Tool).

We need more info from you about which entry method you are using.

A quick “shot in the dark”:

De-select the option “Check for Extra Notes”.

Also, take a look here:

http://www.specialmillwork.com/finaletips/Tuplets%20with%20Simple%20Entry%202.pdf 

I've tried both the Simple Entry, and Speedy Entry, Tools - same result. And I'm reasonably sure I was able to do this in previous versions.

I want to create a quarter note triplet, but the last quarter must be divided into 2 eighths of different pitch. If I used the Simply Entry Tool and select Quarter Note and Triplet Tool, I can create the first two quarters under the triplet bracket, but the remaining quarter rest "placeholder" only expects a 3rd quarter note. If I select an eighth note from the palate, It will allow me to add it but it doesn't believe there is room for the 2nd eighth note - it seems like it cannot subdivide the 3rd quarter into two equal notes. If I then move to the Selection Tool, Finale completes the tuplet with a sixteenth rest!

If I choose to create the equivalent triplet of 6 eighths in the space of 4, I can create the equivalent "sounding" triplet by tying the first two eighths into a quarter, and tying the 3rd and 4th eighths into a quarter, and follow with the 5th and 6th eighths on the separate pitches that I need. But it doesn't LOOK the way I want it to look, again as a quarter-quarter-two eighth mixed tuplet. 

I found a solution! The problem was that I was trying to create the tuplet as the SECOND tuplet of the 4/4 measure. If I create it in a separate measure, or as the first thing of the measure, it will accept the 2nd eighth note. Similar to other issues with end-of-the-bar triplet creation.

Lesson learned...

De-selecting “Check for Extra Notes” will allow you to end a measure with a tuplet.

Where is the option check for extra notes? Can't find it.

It depends on which note entry you're using.

For simple note entry, in the pulldown menu you'll see a choice of "Simple entry options". You'll see "check for extra notes" when you choose this option.

With Speedy entry, "check for extra notes" is right in the speedy's pulldown menu.

I have to disagree. You don't have to deselect anything.

Enter the first quarter note.

Tap Opt/Alt-9, depending if you are on Windows or a Mac. Set the tuplet definition as 3 quarters in the space of two quarters. Save as default.

When you hit OK, you will go back to the score, where you enter the second quarter.

Tap the 4 key to switch eighth note, and enter the last two eighths.

Now, you can enter the sequence as:

5, note.

9

5, note.

4, note, note.

Saving the default as I described it will let you enter any sequence of eighths and quarters, on any beat.