Warm welcome to the forum.
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Grapes are not great grown in a container. If you do grow them in a container you need a very large container - 24".
Plants will drop flowers and fruit when they are stressed - poor soil or too little or too much water.
You need to either plant in the ground or in a MUCH larger pot. Not knowing the variety of grape or your regional conditions it is difficult to recommend a soil mixture.
Pot them up using an all purpose soil mixture. Monitor your watering practice.