I am also having trouble calibrating my fpv in DJI Assistant 2 (DJI FPV series). I have the most up to date version of the program. Once I get the drone plugged in and the system going, the "Calibrate Now" button stays greyed out and unclickable and the loading wheel next to it keeps spinning.

Hi, Tony3004. Can you please follow the steps below and see if it will help?
(1) At this moment, the vision drive is abnormal. Please find a drive named “VISION” or a drive with a yellow exclamation mark in “Device Manager” and reinstall it.
(2) Open the “Device Manager” and open each drop-down menu. Find the drive named “VISION”. Right-click and open it. Select “Update Driver Software”.
(3) Select “Browse the computer for driver software” and tap “Browse” to find the Drivers file folder under DJI Assistant 2 software installation directory  
(4) Select “Next” and the drive will start installation until the prompt for installation completion appears. After the drive installation has finished, the yellow exclamation mark in the Device Manager List will also disappear.
(5) Go to Device Manager and check whether the drive has an exclamation mark (for example C1, MTP, etc.) Use the same method for reinstallation.
(6) After installation, restart the computer and reconnect the DJI Assistant.
(7) If the issue persists, we recommend using another computer.
Have a nice day.