What is the main difference between chā and chà?

See a translation

They have the same meaning when used as verbs.
chā can be used as nouns (means the difference, the result of subtracting two numbers, error, deviation) .
the difference：差别。差距。差额。差价。
the result of subtracting two numbers：差数。四减二的差是二。时差;岁差;剪刀差;顺差,逆差
error：差错。偏差。差池。
chà can be used as adjectives（not equivalent, not good, not standard enough)
not equivalent：差不多
not good：差劲。差等。成绩差。

还有，chà也可以作为名词，但很少见。我也是为了回答你的问题查新华字典才知道他可以作为名词。
chà also can be used as nouns, but very rarely. I looked up the Xinhua Dictionary to answer your question before I knew it could be used as a noun.