There are multiple reasons you might not be able to mount your file system on your EC2 instance. For a list of possible mount issues and resolutions, see Troubleshooting mount issues.
You can also follow the steps in the AWSSupport-CheckAndMountEFS AWS Systems Manager Automation document to check if the instance meets the prerequisites to mount the EFS on the EC2 instance. To run this automation, follow the instructions in Running a simple automation.