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Can I say: " I couldn't able to be a singer"??

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No, because able is an adjective, not a verb.

CB

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Thank you so how I suppose to say?

"Thank you. What should I say?"

I'm not sure what you are trying to say. Perhaps:

I couldn't be a singer. / I wouldn't succeed as a singer. / I wouldn't be able to make a living as a singer.

CB