You should fill out the life insurance application form clearly and honestly. Do not hide pre-existing condition like Diabetes, Hypertension or pre-existing diseases like heart or kidney.
If you do not lie on the application, and disclose all health issues, there should not be cause the the life insurance company to deny a claim to pay on your life insurance, whether you bought the life insurance policy online or off-line.
If the applicant does not disclose a material fact that would affect the underwriting of a life insurance policy, then there may be cause for a life insurance claim to be denied if the death occurs within the first two years of being insured by the life insurance policy.