Monitors are passive devices and therefore require a graphics card to output compatible signal to enable variable refresh rate features.

So the first step we would like to suggest is to check if your monitor is listed as compatible on the graphics card website. For FreeSync, please check it here:

https://www.amd.com/en/products/freesync-monitors

How to enable FreeSync through graphics card settings:

https://www.amd.com/en/support/kb/faq/dh-013

If you have an NVIDIA graphics card, please check the G-Sync compatible monitor list below:

https://www.nvidia.com/en-us/geforce/products/g-sync-monitors/specs/

How to enable G-Sync via graphics card settings:

https://www.nvidia.com/content/Control-Panel-Help/vLatest/en-us/mergedProjects/nvdsp/To_use_variable_refresh_rates.htm

On the monitor, please turn off DyAc/DyAc+ (if your monitor has that feature) through the monitor menu, since DyAc/DyAc+ is not compatible with variable fresh rate technology.