Modify text of label of GUI not working

My GUI has a fuel level that displays the integer amount of fuel that the block contains. This works fine, when I enter x amount of fuel into the block it shows the correct amount of fuel in the block.

I've done this by putting the following into the label: Fuel: <BNBT:integer:fuelAmount>

Alright?

So why I can't I do the same for text / string variables?

Like if I put: <BNBT:string:EXAMPLE> or <BNBT:text:EXAMPLE> it throws an error.

The plan is to have a progess bar like so: [||||||||||||||||||||] that fills up over time. I am capable of doing so if MCreator would actually allow me to use a string variable.