The most common reason you will be unable to change the end user on an Individual or Designated Computer license is because validation is enabled on the license but the validation status is overdue for one or more activations.
You can force validation from the MATLAB toolstrip. Open the Help menu, then select Licensing > Update Current License. An internet connection is required to update validation. If you are unable to validate the activation, then the activation in question must be deactivated from the MathWorks License Center:
Once validation has been updated or the activation in question has been deactivated, you can change the end user on the license:
Also, you may be unable to change the end user on your license because you are out of Named User re-designations on your license. Each license has an amount of times you can change the user per year. For further information on this please refer to the license agreement or contact Licensing and Installation Support. The X under the "Remove User" column may not appear when removing a License End User if you are using an older web browser version. If you are encountering this issue, place the mouse cursor under the "Remove User" column and click to remove the user or try another web browser.