who is not eligible to enter into a contract and why

Minors (individuals under the age of 18 in most jurisdictions) are generally not eligible to enter into a contract because they lack the legal capacity to understand the terms and implications of a contract. Additionally, individuals who are mentally incapacitated or under the influence of drugs or alcohol may also be deemed ineligible to enter into a contract due to their inability to fully comprehend and consent to the terms of the agreement.