Learn Spanish (Latin America): No es que busque una excusa, sino que tengo pereza. - It is not that I am looking for an excuse, but rather that I am feeling lazy.

The phrase no es que introduces a rejected reason or a false assumption, which triggers the subjunctive. By using busque, you are explicitly denying this action and saying "It is not the case that I'm looking for an excuse."

Why is tengo in the indicative, while busque is in the subjunctive?

While the first part of the sentence rejects a reason (using the subjunctive), the second part after sino que states the actual, true reason. Because you are declaring a reality ("I am actually feeling lazy"), you must use the indicative verb tengo.

Why do we use sino que instead of just sino?

We use sino que because the contrast introduces a new clause with a conjugated verb (tengo). If you were only contrasting a noun or adjective without adding a new conjugated verb, you would just use sino.

Why does the sentence use tengo pereza instead of an adjective like estoy perezoso?

In Spanish, many physical or mental states are expressed as nouns you "have" using tener, rather than adjectives you "are" using estar. Tener pereza literally translates to "to have laziness" and is the most natural way to express that you are feeling lazy.