I'm still not fully convinced with the paragraph (many flaws). But, since you asked particularly about those words and the articles, I'm addressing that so that to clarify that in general. Maybe, you take care of this in future then.

In this case, you are talking about the challenge of lacking something. Thus, it's the challenge and not two or more challenges!

In any case, criteria will not have a, it's plural. The singular will be criterion.

I'm still not fully convinced with the paragraph (many flaws). But, since you asked particularly about those words and the articles, I'm addressing that so that to clarify that in general. Maybe, you take care of this in future then.

In this case, you are talking about the challenge of lacking something (the word lack in your sentence is used as a singular and countable). Thus, it's the challenge and not two or more challenges!

In any case, criteria will not have a, it's plural. The singular will be criterion.

I'm still not fully convinced with the paragraph. But, since you asked particularly about those words and the articles, I'm addressing that.

If you consider criteria and requirement as one component, it becomes the challenge and not challenges. If you consider them separate in your actual scenario, they take plural.

In any case, criteria will not have a, it's plural. The singular will be criterion.

I'm still not fully convinced with the paragraph (many flaws). But, since you asked particularly about those words and the articles, I'm addressing that so that to clarify that in general. Maybe, you take care of this in future then.

In this case, you are talking about the challenge of lacking something. Thus, it's the challenge and not two or more challenges!

In any case, criteria will not have a, it's plural. The singular will be criterion.