Hey, @lizzysemailb14_ESO. To fix this error, please bring up your machine's Task Manager (ctrl+alt+del) and ensure that any and all processes relating to The Elder Scrolls Online: Tamriel Unlimited are ended. Do this before you run the launcher. Please be sure to take special care to end a process named "ZoCrashReporter", as this is the most likely offender preventing you from successfully repairing the client or reinstalling your launcher.
If you ended all associated tasks, and then tried repairing the client or reinstalling the launcher, you should be able to proceed.