Hello @Peter Goossens,
Welcome to Microsoft Q&A forum.
Is that older project based on .NET Core? Maybe .NET Core 3.0, 3.1?
I suggest firstly close VS, open VS Installer, click Modify, switch to Individual components and search for key word .NET Core
. Since old framework like .NET Core is out of support, you may need to install it manually/separately.
If it doesn’t work, please try to download and install the .NET Core from here: Download .NET(scroll down and expand Out of support versions).
Please also try to close the solution(File > Close Solution) and reopen your project, see if the issue disappears.
Best Regards,
Tianyu
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