Good question, I believe you should be able to, if you can heal, it must mean you have resources Not having resources was my only idea.

Wait up .Do you mean the same tier, or a higher tier? I believe training higher tier troops isn't straight forward, ie  it isnt just about having resources.

Build a barracks, if you have not already done so.  Click on it and then on "troops".  The screen shows you all the troops which exist, those for which you do not yet have the knowledge to train are padlocked.  Click on one that is not chained and move the slider to specify the number of troops you wish to train - the screen shows you how long the training will take , more troops longer time.

You might take a look at the barracks section of the buildings guide.

https://www.vikingswarofclans.com/game-guide/military-buildings-guide/