CS1929 'UniTask' does not contain a definition for 'Delay'

For some reason, it is not possible to access the "Delay" method in my project. I have installed the latest version of UniTask.

Severity Code Description Project File Line Suppression State Error (active) CS1929 'UniTask' does not contain a definition for 'Delay' and the best extension method overload 'MECExtensionMethods2.Delay(IEnumerator, TimeSpan, Func<TimeSpan, bool>)' requires a receiver of type 'System.Collections.Generic.IEnumerator' ScpApiClient C:\Users\USER\Documents\Visual Studio 18\SCPSL Plugins\TamagoServer\ScpApiClient\DataModels\User.cs 558