The word 'uncertainness' is correct and can be used in written English.
You can use it to describe a situation in which a certain outcome is unclear or unpredictable. For example: "The uncertainty of her job prospects left her feeling anxious and uncertain."
Dictionary
uncertainness
noun
The quality of being uncertain.
synonyms
Exact(2)
Havrilesky and others (there's the anonymous Dear Coquette, and Mallory Ortberg, who is Slate's new Dear Prudence) understand the uncertainness of modern life.
We would therefore question how the committee can make an informed judgement now that the 2025 target will be in line with our EU and global competitors, when significant uncertainness remain".
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Since I tried Ludwig back in 2017, I have been constantly using it in both editing and translation. Ever since, I suggest it to my translators at ProSciEditing.

Justyna Jupowicz-Kozak
CEO of Professional Science Editing for Scientists @ prosciediting.com