user must click all buttons before "next" appears?

Simple solution-

Whenever you want someone to select MULTIPLE items before an action can occur, you ALWAYS use letters added to an existing variable. Remember to ONLY add the letter value to the variable IF the variable does not already include the letter. All these previous solutions would be adding a hundred "a"s to a variable if the A button was clicked one hundred times.

So: First ACTION on each button:

On: Click
Action: Modify Variable
Target: myVariable
Value: a
Modification Type: Add to Variable

IF myVariable DOES NOT INCLUDE "a"

Second ACTION on each button:

On: Click
Action: Run Action Group
Target: GRP_WhateverNameYouWant

Create an ACTION GROUP called "GRP_WhateverNameYouWant" that includes a list of actions, the first being to execute an action (Show, Hide, Whatever) IF the variable myVariable meets multiple conditions i.e. myVariable contains "a",myVariable contains "b", etc.

If the conditions are not met you can use the THEN Tab (of the IF/THEN combo) to perform some other action - OR -

you could use a list of other actions to perform other actions based on other conditions. In other words, if myVariable contains "a" + "b" but NOT "c" then you could perform an action unique to those circumstances.

The reason to put these actions in a GROUP is so that you can "share" the actions with other buttons, and add other conditions if required.

You NEVER want to check for a "string sequence" like "abcde" because the learner might select "dbcab", AND because the previous solutions listed here might have ended up with a single variable value of "abbdcaaddcbaaeddcceda" if the user selected buttons multiple times (which they ALWAYS do). Use MULTIPLE conditions that check for a single value : variable contains "a", variable contains "b", variable contains "c", etc.