How to show monster ac in token bar?

I tried setting default bar 2 value to 'ac', 'npc_ac', '@{npc_ac}', '@{selected|npc_ac}', but none of these cause any value to be put into the token when I create a new one from a journal monster. I saved settings and rejoined game each time. What am I doing wrong?

You need to link the bars, not set their default value. There should be a dropdown to let you choose the attribute to link to. 

There are two areas where you can do this (assuming you are talking about the D&D 5th Edition by Roll20 Sheet) on the Campaign settings page. You do not want to use this one: You want to use the section that is dedicated to the sheet at the bottom: The settings above will put in HP values but not link them in bar 1, and will link Speed in bar 2 and AC in bar 3. You don't want to link HP on a monster token, or all goblins on the board will share the same current HP.

I have the setting as shown here but neither bar 2 nor 3 works, for tokens from compendium and for characters created after changing the settings. What am I missing?

You should apply in-game settings: Go to the settings tab on the right and scroll to the bottom where you will find a button that says Apply Default Settings. Press it. Choose to update the Token settings. Wait a minute or two for it to resolve. There is a thin easy-to-miss progress bar. But that should only apply to in-game content already on the board. Just changing the settings should have been enough for new compendium drops. Also: Make sure you are not dropping something that is already in your game. If your goblins are set incorrectly, no amount of compendium dropping will give you corrected goblins because the compendium will not overwrite in-game content (to protect modifications and home brew). If you suspect this is the case, you will need to delete or rename any existing "goblins" (or whatever) to get the new defaults. properly set up.

Thank you for your efforts, but I am still having problems. I applied default settings, and dropped in an air elemental from the compendium, which have not been in the game at all til now, and it is not showing ac: The hp is only working because of an API script for auto-rolling.

I'd suggest going through the troubleshooting steps I'll put below the line, then capturing a Console Log ( Chrome ,  Firefox ) from the time during a drop, and filing a  Help Center Request . Could you please carefully work through these steps of our guide to  Solving Technical Issues ? Step 1: Make sure to use the right browser Please check if these issues persist when using both Chrome  and  Firefox. Step 2: Ensure that there are no extensions/addons interfering with Roll20 Please disable  all  add-ons/extensions. Step 3: Clear your cache If none of the above succeeds, please thoroughly work through  Step 4 .  (Don't forget the complete Console Log!) Thank you!

How do you get to the second set of settings. I only have the first set when I click on the token and the settings button.

The second set of settings are the sheet settings. You get to that by clicking on the "Settings" dropdown to the right of the "Looking for Players" button. Select "Games Setting" from the dropdown. Click on "Game Default Settings" Scroll down to the Character Sheet Template section and make sure you have the "D&D 5E by Roll20" active. Scroll down further and adjust the defaults for the bars. Be sure to save. You might also want to check your Compendium settings that they are "D&D 5th Edition (Character Sheet Default)" Also, be sure to save.

In order to actually get a bar to appear (rather than just having a value appear in the circle), both the "value" and "max" settings need to be numeric values.  Unfortunately, it doesn't seem to be possible to use an armor class attribute for this since AC (unlike HP) doesn't have "current" and "max" secondary properties to link to the "value" and "max" elements.  I tried using the Game/Settings page to use "ac" and "npc_ac" for both the "value" and the "max" for a bar but was unsuccessful.  I suspect the only way around this will be with a macro or an API script.