There are a few possibilities here:
If you have an individual license, it is possible that you somehow installed Simulink but you do not have a license for it. If this is the case, you will need to purchase a Simulink license and then use Help -> Licensing -> Update Current Licenses
If you have an individual license, it is possible that you somehow installed Simulink but your active licenses do not reflect the Simulink license that you purchased later than you installed MATLAB. In such a case use Help -> Licensing -> Update Current Licenses
If you have a concurrent license, it is possible that you installed Simulink but the concurrent license does not cover Simulink. If this is the case, the license administrator will need to purchase Simulink licenses and then update the license file, and then you will need to exit MATLAB and reenter to gain access to the newly-entered Simulink license.
If you have a concurrent license, it is possible that you installed Simulink but there are not sufficient concurrent licenses to cover your use. In such a case, you can either wait for a license to become available, or you can arrange with the license adminstrator to purchase additional licenses and update the license file.
There is an additional possibility: You might have recently added a trial license to an existing MATLAB installation, and the trial license does not happen to cover Simulink, and then you purchased a Simulink license and activated it. In this combination of circumstances, the trial license might take priority over the regular licenses until the expiration of the trial licenses. To deal with this circumstance, you need to find and delete the trial license file and restart MATLAB.
