'Can't access classes of Unity

Here, when I open the highlighted script in a Unity Project: enter image description here The visual studio opens and there's no solution explorer: enter image description here and even if I add it from View tab, there's nothing in it to be accessed or browsed.enter image description here

Due to this I'm not able to access Unity classes like in this example Vector3 class cannot be accessed which can be noticed as it's not color coded.enter image description here How can this problem be solved?



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