The Pre-Check button remains disabled (greyed out) after a .mub file upload.
Re-uploading the .mub file does not change the behavior
VMware NSX
The Pre-Check greyed out is an indication that the .mub file does not contain the scripts and logic for pre-check, and requires a .pub file
Click on the upgrade button to advance to the next screen, where you will be requested to upload the respective .pub file.
Once the respective .pub file is upload, the pre-check button should be enabled to run the required checks.