Support, Treatment, and Overdose Prevention of Fentanyl Act of 2021 or the STOP Fentanyl Act of 2021 This bill addresses data collection, treatment, harm reduction, and other issues related to substance misuse and addiction. It particularly focuses on opioids, fentanyl, and related substances. Specifically, the bill expands surveillance of fentanyl and related substances. This includes establishing a pilot program to screen for contaminants in illicit drugs and a grant program to collect data on fentanyl-involved overdoses. The bill also specifies requirements concerning the collection and use of information about drugs seized by law enforcement. Additionally, the bill establishes and modifies requirements to increase access to treatment and prevention services. For example, the Bureau of Prisons must make certain services, including medication-assisted treatments, available to individuals in its custody. The bill also repeals a provision that requires dependence on opioids for at least one year before an individual may be admitted to certain treatment programs; allows providers to prescribe medications to treat substance use disorders following a telehealth evaluation; creates grant programs for treatment and harm reduction activities; and addresses issues pertaining to opioid overdose reversal drugs. Furthermore, the bill limits civil and criminal liability under specified laws for individuals who administer opioid overdose reversal drugs to an individual who reasonably appears to be experiencing an overdose, as well as for individuals who seek medical attention for themselves or on behalf of another for a drug overdose. The bill also requires multiple reports, including a report by the Department of State on foreign sources of certain drugs.