How to enable Realview graphics (Intel) Solidworks 2021

At the top of your screen of SW, there is a "monitor" icon, that turns on and off REALVIEW GRAPHICS with the drop down arrow. IF your part does not change and doesn't look different.....then your graphics card does not support REALVIEW. You can double check your graphics card compatibility on Solidworks website. If it has worked in the past there might also be a graphics driver update for your card.

If by 2020 ver you're able to use it, then it could be the Software's problem of not being able to read your graphic setting.

Not sure what have you tried before, but here are method you may try:
1. Re-install/update graphic driver by (search Device manager>display adaptors>right click on the graphic drive and try to updates or uninstall/reinstall. Bare in mind, the uninstallation will require you to restart the PC to make sure thing reset properly, so make sure you're trying this while nothing running in the background)

2. Copy the previous Solidworks version's setting and make it for current version:
Step 1:
Search in your windows search engine > Regedit

Step 2:
Regedit > HKEY_CURRENT_USER > Software > SolidWorks

Step 3:
SolidWorks > right click Solidworks 2020, select copy and paste as new name

**As you say you had Solidworks 2020 before, under your Solidworks should have 2 different Solidworks name, such as Solidworks 2020 and Solidworks 2021.
They're registry file containing your Solidworks setting with respect to different version
Thus make sure when we try use back this 2020's setting for 2021, the name must follow how the system put. For me, my system set my Solidworks 2021 to have a name as solidworks 2021, so if i copied the Solidworks 2020's setting, i need rename it as solidworks 2021.

Step 4:
Re-run your solidworks and test again.

3. Last suggestion, force Enable the usage of graphic driver for Solidworks via system registry, as follow:

Step 1:
Search in your windows search engine > Regedit

Step 2:
Regedit > HKEY_CURRENT_USER > Software > SolidWorks

Step 3:
SolidWorks > Solidworks 2021 > Performance > Graphics > Hardware > Current
**As you had Solidworks 2020 before, so make sure you're selecting the correct Solidworks version that unable to activate realview graphic

Step 4:
Copy your graphics card name and the key under "Workarounds"
e. g. “Intel(R) HD Graphics 520” and "4000080"

Step 5:
Hardware > G12Shaders > Other

Step 6:
Presuming your graphics card isn’t any of the ones listed;

Right-click “other” > New > Key

Step 7:
Rename “New Key #1” as your exact graphics card name as copied,
e.g. “Intel(R) HD Graphics 520”

Step 8:
Right-click the new folder you just made > New > DWORD (32-bit) Value

Step 9:
Rename “New Value #1” as “Workarounds”

Step 10:
Right-click “Workarounds” > Modify… > and put your key e.g. "4000080" as the Value data (with the base in Hexadecimal) > OK

Step 11:
Start Solidworks > Enable Realview Graphics