Adjusting status to become a Legal Permanent Resident while in the United States on a Visa Waiver requires careful thought and analysis.
USCIS and the US State Department is particularly sensitive to matching a persons true intent when entering the United States with the appropriate type of visa or special entry permission, like a Visa Waiver. It is an absolute “no no” to use a Visa Waiver to enter the United States for the purpose of getting married and applying to become a Legal Permanent Resident (immigrant). To do so would likely constitute fraud. The visitors intent at the time of entry is the issue.
If a person enters the United States as a visitor under the Visa Waiver program and truly intends to visit with no agenda to get married and then after entering the United States, thereafter decides to get married and wishes to stay, applying for Legal Permanent Residency is a permissible option.