Elizabeth, usually this happens when your text is very long or contains too much formatting.

When you highligt two letters, the actual code can be like this:
"When you highligt two let<span style="background-color: yellow;">te</span>rs". This makes your text longer.

What you can do is split your correction in two parts and send both as two corrections:) 

Thank you so much, tano. I can now try to pretend that I didn't waste my entire morning.

I recently had that problem too. I also posted a similar comment to yours and discovered that this is quite a common problem.

What about when trying to create a new lesson? My submit button won't allow me to get through