I have solved my problem, it was an implementation mistake of mine, you have to declare a variable instead of assigning a value, example:
class IUnity:NSObject, UMSInitializationDelegate, UMSInterstitialAdLoadDelegate, UMSInterstitialAdShowDelegate, UMSRewardedAdLoadDelegate, UMSRewardedAdShowDelegate{
static let AppID:String = "XXXX"
static let FullID:String = "video"
static let MoneyID:String = "iOS_Rewarded"
var general:AdsGeneral?
var isNew:Bool?
var rewardedAd:UMSRewardedAd?
[ var interstitialAd:UMSInterstitialAd?
..........
interstitialAd = UMSInterstitialAd(adUnitId: IUnity.FullID)
interstitialAd!.load(with: self)