I've seen people post about subbing a T in for TE on run plays.

Where does one make such play-specific substitutions? Can't seem to find to where.

Go to depth chart and the overrides and you can change any position on any formation

I just get an error message if I click a player box.

When on the override page, click and drag the player you want to put into that position from your roster. After doing this, the position will turn red, meaning you have an override for it. You can use multiple overrides for each position.

Note, you must click and drag the player from the roster. If you try and move a player that is already in the "override position", you will get an error message.

This isn't "play-specific" though. It's formation specific. But if you only call plays where the TE stays in to block in certain formations, then this is a good idea.

EDIT:
It's actually formation and down and distance specific.

Thanks guys!