How to create a conditional text field

Not sure what exactly you mean by "textbox".

If you are talking about the region called "Text Box" by PTC Prime, the answer is "no". These Text Boxes can only be created manually by pressing Ctrl-T and typing the text.

Of course you could use a small program sequence using the if-statement to return a text string. But the program would be visible in any case and the the string at best could be an empty string.

You could hide the creation of the text in collapsed region, but in case that no warning is to be issued, you would still see

or any text indicating that all is OK.

You can also use a scripted "Text Box" (-> Advanced Controls) and its not really clever that PTC had chosen the same name for them as for the text region.

Here you have more control concerning font, font size, font color or background color. Examples for this were posted here in the forum a couple of times. But it still applies that in case no warning is necessary, that Text Box won't vanish. It could display a different text or no text at all but it still would be here.

You could use an expression like:

The variable will have the value of Excelvalue if that is not zero, otherwise its value is a string "Warning!" and any numerical calculations your sheet may try to do with Variable will throw an error, unless you check for a variable being a string. Some example:

But if you go this far, then it's just as easy (or even easier) to check for a zero value of Variable before attempting calculation.

Your choice.

Success!
Luc