There are various reasons for the error, this article gives you the start.
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Why automate? I do automated testing because there's only so much a human being can do and remain healthy. Sleep is a requirement. So, while people sleep, automation that I create does what I've described above in order to make sure that nothing gets past the final defense of the testing group.
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To second what @tristaanogre said there is great documentation for tracking down object identification issues.
If you need help even after reading through that we will need a lot more information than an image of the error, code samples, namemapping screenshots and object browser screenshots at the minimum.
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