Hello @Emre Kömürcü,
Welcome to Microsoft Q&A forum.
Please try to reload your solution(File => Close Solution => select and open your solution again).
If it doesn’t work, try to right-click your solution => Restore NuGet Packages.
Besides, restarting Visual Studio maybe also work.
BTW, please kindly edit your question into English, so that we can better help you:)
Update1:
Tested locally on two machines(with the same latest version) and got the same error on one of the machines.
Repairing VS makes this error disappear. Please try to repair VS from VS Installer => More => Repair.
Feel free to let us know the results.
Sincerely,
Tianyu
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