Hi Esther, and Welcome to the Forum!
Some flash drives have a small write protect switch on the side which you must flip. Sometimes the thumbdrives have to be plugged into the computer to activate or deactive it.
Her is an image of a flash drive and switch. Switches can be as shown or on the side.
There are also a couple write protect software programs that are used to protect flash drives. If your drive was protected that way, you need the software to unprotect it.