Regex Class library class declaration error ?

In the program to convert Vietnamese from accented to unsigned in C#, use the code in:

https://www.dtmi.net/chuyen-tieng-viet-co-dau-sang-khong-dau-trong-c-javascript-php-va-sql-server/
https://docs.microsoft.com/en-us/dotnet/api/system.text.regularexpressions.regex?view=net-5.0

I declare the library using System.Text.RegularExpressions; and added the library System.Text.RegularExpressions.dll but still got the error like in the picture https://www.mediafire.com/file/fwmaqggcv5ik7ct/err03Regex.jpg/file