Can I cash a government check at a bank without having a bank account anywhere? It is a substantial amount of backpayment from SSDI.
A bank needs to make sure they won't lose money by cashing a check. When you have an account with a bank and you cash a check, if the check ends up not getting paid, the bank will take the money back out of your account. This could happen for a number of reasons: the check could bounce (not enough money in the check writer's account), it could be a fraudulent (fake) check, or the payment could have been stopped on that check.
Treasury checks are more problematic for banks than private checks; the government has given themselves more power to refuse to pay a check than the average person has. As a result, banks are already overly cautious about cashing treasury checks. The fact that you have a big check increases the risk for the bank.
You'll have to ask around at different banks to see what they will do for you and what type of fee (if any) they will charge. Some banks might cash it for a fee; others might require that you open a savings account and wait a certain number of days after depositing the check before withdrawing your money.