I want to write a superscript in Label text but I Can't see HTML format property of the label in mit app inventor

<sup>.2</sup>

It is probably not supported, HTML support for Labels is not expansive.

ProfessorCad: Tips & Tricks HTML Supported by Label Component

You might like to try Unicode instead, or perhaps there is an extension that delivers a smarter Label.

That tag is acceptable.

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Check you have your Blocks Toolkit set to Default