Which English form is correct:
in an effort to or in effort to ?

"In an effort to" is the correct phrase, not "in effort to."

in an effort to

It's correct
If you are explaining that you did something to try to bring about a result, you can use this phrase. More...

in effort to

It's not correct. Don't use this phrase.
This phrase is missing the indefinite article before the noun "effort."

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