Check your testnet Dash Core wallet version, it should be version 0.17.00 RC3 --> https://github.com/dashpay/dash/releases/tag/v0.17.0.0-rc3
You can check your version by going to Tools - Information within your running Dash Core wallet.
* If it is not latest version then close your Dash Core wallet and update it by replacing the dash-qt.exe with that from the downloaded and extracted win64.zip file
Close your Dash Core wallet
Go to your testnet3 folder and delete the peers.dat file in there (a new one will be created upon start of your wallet)
Start your Dash Core wallet