Installing

The Solver add-in is not loaded by default but should be installed as part of the normal Excel installation.
To load this add-in select (File > Options > Add-ins).
At the bottom next to Manage, select Excel Add-ins and press Go.
Select the add-in called Solver Add-in and press OK to install it.

When this add-in is loaded, the "Solver" command will appear in an additional Analyze group at the end of the Data tab.
Data Tab, Analyze group, Solver

Solver - Displays the Solver Parameters dialog box.

Add-in Location

Excel 365 - C:\Program Files\Microsoft Office\root\Office16\Library\SOLVER.XLAM | SOLVER32.DLL 
Excel 2024 - C:\Program Files\Microsoft Office\Office16\Library\Solver\SOLVER.XLAM | SOLVER32.DLL 
Excel 2021 - C:\Program Files\Microsoft Office\Office16\Library\Solver\SOLVER.XLAM | SOLVER32.DLL 
Excel 2019 - C:\Program Files\Microsoft Office\Office15\Library\Solver\SOLVER.XLAM | SOLVER32.DLL 
Excel 2016 - C:\Program Files\Microsoft Office\root\Office16\Library\SOLVER.XLAM | SOLVER32.DLL