Hi Alisha, This is Ramesh.
To get the "Install" option in the Store app for Opera GX, please do the following:
Launch Regedit.exe and go to:
HKEY_CURRENT_USER\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall
Select the Opera GX subkey -- e.g., "Opera GX 82.0.4227.44"
Click File → Export and save it to a REG file.
Right-click on the Opera GX subkey and choose Delete.
Exit the Registry Editor.
Close Store and reopen it.
See if you can install Opera GX.