Hi @KristianLF ,
Do you mean that if a qualified file is available in OneDrive, then add this file to the Planner task?
If so, then your configuration in Condition should look like this:

In addition, the difficulty in configuring this flow is that it cannot obtain the file link, the file link required in task details, and the existing functions in the OneDrive connector cannot achieve the need to obtain the file link.
Please refer to the methods below to achieve your needs.
- Use Create share link to create a link to access the specified file and save the file name and Web URL as an array.
Image reference:

Details in Apply to each:

Best Regards,