Welcome to Microsoft Q&A! Thanks for posting the question.
I will suggest you to verify that you are passing the file id (not the file path) for the Get File Content action. If you are using file metadata using path action then you need to specify the file path.
In case, if you are passing the right value then please verify if you are using any of the below SFTP servers as the connectors currently don't support them as documented here.
- IBM DataPower
- MessageWay
- OpenText Secure MFT
- OpenText GXS
- Globalscape
- SFTP for Azure Blob Storage
- FileMage Gateway
If you are using an IBM SFTP server then the issue is caused by some default setting on SFTP server side. You can increase ExtractCount to at least 2 if the files are small (no chunking). Otherwise, it will need to be more than the number of chunking : https://www.ibm.com/support/pages/message-extractable-count-set-1-default