Just wait a while. I see this often with various activities, not just Else If. Whatever background process is doing the on-the-fly validation is sometimes very slow. While you’re waiting, if you run/debug the error icon will disappear once the validation completes.
Also, I’ve noticed that the Else If activities make Studio go nuts with memory usage. We are on Studio 2023.4.4 and UiPath claims it’s fixed in later versions. We will see.