In this article you will learn the solution to the common error "Only one <configSections> element allowed".
Today I was working on Entity Framework and trying to add the connection string to the Web.Config to specify the database. I wrote the connection string like this: 

  <?xml version="1.0" encoding="utf-8"?>     
    <configuration>   
       <connectionStrings>  
          <add name="SQLConnect"
               connectionString="Data Source=SAHIL; Initial Catalog=Demo; Integrated Security=SSPI"
               providerName="System.Data.SqlClient" />  
       </connectionStrings>    
       <configSections>  
          <sectionnamesectionname="entityFramework" type="System.Data.Entity.Internal.ConfigFile.EntityFrameworkSection, EntityFramework,
              Version=6.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" requirePermission="false" />
       </configSections>    
       :  
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    </configuration>

When I run the application, I experienced a strange error that says: "Only one <configSections> element allowed. It must be the first child element of the root <configuration> element".

It took me some time to determine the cause of the error and how to fix it.
Error: "Only one <configSections> element allowed. It must be the first child element of the root <configuration> element".

If you read the error carefully, it states that only one <configSections> element is allowed inside the Web.config and it should be the first child element and placed at the top. The reason for the error is that I accidentally placed the <connectionStrings></connectionStrings> at the top over the <configSections></configSections> and by conventions this is a violation. So, to fix the error, I rearranged the elements and the error was fixed.

<?xml version="1.0" encoding="utf-8"?>   
<configuration>  
   <configSections>  
      <sectionnamesectionname="entityFramework" type="System.Data.Entity.Internal.ConfigFile.EntityFrameworkSection, EntityFramework,
       Version=6.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" requirePermission="false" />
   </configSections> 
   <connectionStrings>  
      <add name="SQLConnect"
           connectionString="Data Source=SAHIL; Initial Catalog=Demo; Integrated Security=SSPI"
           providerName="System.Data.SqlClient" />
   </connectionStrings>   
   :  
   :  
   :  
   :  
   :  
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   :  
</configuration>

Conclusion

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