What do I do with a new. Net core application after referencing the item always yellow exclamation mark?

When we create a. Net core application in VS, the reference item may appear with a yellow exclamation point, which, under normal circumstances, can disappear quickly after the project is created, but sometimes it does not disappear.

We can select the exception item and then the right-click menu and choose the regenerate yellow exclamation point will disappear.

After rebuilding:

What do I do with a new. Net core application after referencing the item always yellow exclamation mark?