問題：
I'm following the docs in order to create an initial migration.我正在關注文檔以創建初始遷移。 When I execute dotnet , I get the help section, meaning that the PATH works properly.當我執行dotnet ，我得到了幫助部分，這意味着 PATH 工作正常。
Then I try to execute the command below from the docs in console window:然後我嘗試從控制檯窗口中的文檔執行以下命令：
dotnet ef migrations add InitialCreate dotnet ef 遷移添加 InitialCreate
I get the following error:我收到以下錯誤：
Could not execute because the specified command or file was not found.無法執行，因爲找不到指定的命令或文件。
Possible reasons for this include:可能的原因包括：
You misspelled a built-in dotnet command.您拼錯了內置的 dotnet 命令。
You intended to execute a .NET Core program, but dotnet-ef does not exist.您打算執行 .NET Core 程序，但 dotnet-ef 不存在。
You intended to run a global tool, but a dotnet-prefixed executable with this name could not be found on the PATH.您打算運行一個全局工具，但在 PATH 中找不到具有此名稱的 dotnet 前綴的可執行文件。
I excluded the first item since I copied the command.自從我複製命令以來，我排除了第一項。
I excluded the second item because the package Microsoft.EntityFrameworkCore.SqlServer is installed.我排除了第二項，因爲安裝了Microsoft.EntityFrameworkCore.SqlServer包。
I excluded the third item because I get the help section when invoking dotnet .我排除了第三項，因爲我在調用dotnet時得到了幫助部分。
I'm googling the issue but since the version is new, there's not much to go on and/or it's drowning in similar issues from earlier versions.我在谷歌上搜索這個問題，但由於版本是新的，所以沒有什麼可做的和/或它淹沒在早期版本的類似問題中。
I tried to forcibly install Microsoft.EntityFrameworkCore just in case it needs to be explicitly added.我試圖強行安裝Microsoft.EntityFrameworkCore以防萬一需要明確添加。 I ran into the error message telling me that the latest version to pick from is 2.2.6 and a downgrade is a no-go.我遇到了錯誤消息，告訴我要選擇的最新版本是 2.2.6，不能降級。 I'm not sure how to install the version compatible with the SQL-package I have on my system already (and even less certain if that's right approach to kill this issue).我不確定如何安裝與我係統上已經擁有的 SQL 包兼容的版本（甚至不確定這是否是解決此問題的正確方法）。
Detected package downgrade: Microsoft.EntityFrameworkCore from 3.0.0-preview6.19304.10 to 2.2.6.檢測到包降級：Microsoft.EntityFrameworkCore 從 3.0.0-preview6.19304.10 到 2.2.6。 Reference the package directly from the project to select a different version.直接從項目中引用包以選擇不同的版本。
Web ->網頁 ->
Microsoft.EntityFrameworkCore.SqlServer 3.0.0-preview6.19304.10 -> Microsoft.EntityFrameworkCore.SqlServer 3.0.0-preview6.19304.10 ->
Microsoft.EntityFrameworkCore.Relational 3.0.0-preview6.19304.10 -> Microsoft.EntityFrameworkCore.Relational 3.0.0-preview6.19304.10 ->
Microsoft.EntityFrameworkCore (>= 3.0.0-preview6.19304.10) Microsoft.EntityFrameworkCore (>= 3.0.0-preview6.19304.10)
Web -> Microsoft.EntityFrameworkCore (>= 2.2.6) Web -> Microsoft.EntityFrameworkCore (>= 2.2.6)
解決方案：
參考一： https://stackoom.com/question/3rRhA參考二： Command dotnet ef not found in .NET Core 3