你的函数代码总是到达最后一行，就像...
如果工作表对象存在，则必须放置一个代码行以退出函数：

Function SheetExists_(SheetName As String, wb As Workbook) As Boolean
On Error GoTo Message
SheetExists_ = Not wb.Sheets(SheetName) Is Nothing
If Not wb.Sheets(SheetName) Is Nothing Then Exit Function
Message:
    MsgBox "Unknown Error"
End Function

已编辑：

Function SheetExists_(SheetName As String, wb As Workbook) As Boolean
On Error GoTo Message
SheetExists_ = Not wb.Sheets(SheetName) Is Nothing: Exit Function

Message:
    'reaching this part will (only) make it returning `False`...
End Function

请注意，上面的函数是SheetExists_。它有一个下划线字符的名称结束。我有另一个函数与此名称...

“SheetExists”函数将始终将“ErrorMsg”设置为“未知错误”。在SheetExists = Not wb.Sheets(SheetName) Is Nothing之后添加“Exit函数

现在的方法是，当工作表不存在时，将ErrorMsg设置为“Unknown Error”。这就是为什么在测试设置中，每张工作表都是不同的，所以每张工作表都会出现错误。您的函数仍然会给予False（不存在），但也会出现错误。

使用简单的函数进行检查，不要将工作表与工作表混淆

Public Function sh_Exist(TheWorkbook As Workbook, SheetName As String) As Boolean
Dim s As String
On Error GoTo errHandler
s = TheWorkbook.Sheets(SheetName).Name
sh_Exist = True
Exit Function
errHandler:
End Function