4.0 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-vi...va-sql-server/
https://docs.microsoft.com/en-us/dot...x?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

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