how to fix an error of erasing a drive on MacBook Pro 2014

I see that the drive is in use by a process and that makes that it cannot be erased

First, startup the Mac from macOS recovery (cmd+R on startup) after that, select disk utility in the recovery menu. Select the disk you want to erase, click erase in the tool bar of disk uitility. After that, you can install macOS anew if this is needed (when this is about your startup drive) via the recovery menu

You don't want to erase the internal boot drive as FAT32 if you still want to boot macOS from the internal SSD.

Usually FAT32 is used for USB sticks or SD Cards. FAT32 is a poor choice for a large SSD or hard drive.

Edit: Why are you using the command line to do this anyway? And what is your goal?