An Activation Key is used to link your MathWorks account to a license that you are not already linked to. A File Installation Key is used for offline installations or if you do not have a MathWorks account. Unless you are trying to link your MathWorks account to a license that it is not already linked to, then you probably don't need an Activation Key.
To verify you have the correct Activation Key, contact your License Administrator(s). If you are the administrator of the license, view the documentation below.
Where can I find the Activation Key or File Installation Key (FIK) for my license?https://www.mathworks.com/matlabcentral/answers/102845
If you see the option to select a license during installation, you do not need to also enter an Activation Key, unless you are trying to link to a new license. You can simply select your license and then "Next".
If you are seeing the screen below with no option other than to enter an Activation Key, then this means your MathWorks account is not tied to any licenses. You either need to purchase a license, obtain a trial license, or contact a License Administrator for a valid Activation Key.