A Section 21 notice is a legal way for landlords in England to evict tenants without giving a reason. However, tenants can challenge it if it is not valid. First, check if your landlord followed all legal steps, like protecting your deposit in a government scheme. Also, the landlord must have given you the required documents, such as an Energy Performance Certificate and a Gas Safety Certificate. If the notice has mistakes or the landlord has broken the rules, you may have a case to challenge it in court. Seek legal advice or contact a tenant support organization for help.