Error detection: I wandered lonely as a cloud that float on high over hills and vales.
Error detection: I wandered lonely as a cloud that float on high over hills and vales.
Explanation
The subject "cloud" is singular, so the verb must also be singular.
"Float" should be "floats" to agree with the singular subject.
Correct sentence: "I wandered lonely as a cloud that floats on high over hills and vales."