If your account credit balance is low (below 150 TRX), you won't have enough to start a new rental. You are required to have a balance that is enough to cover 24 hours of usage even if you don't use the account for the entire time.
Go add credits if you need to.
The KeyStream extension lets you rent accounts for 24 hours at a time.
You will then wait until a seller accepts your request. Once they do, you will automatically be logged into the account and can use it for up to 24 hours. You may also end your rental early if you wish and you will only be charged for the time you used.