Lost Space after Removing Windows using BootCamp

Did you use a non-BCA program to remove the partition? This commonly happens when that is attempted.

The easiest way to fix this, and any partition issues, is to back up your Mac with Time Machine, then erase it and restore from the backup.

1. Back up your Mac with Time Machine - Apple Support
2. Use Disk Utility to erase an Intel-based Mac - Apple Support
3. Restore your Mac from a backup - Apple Support

Also, if the file system of the main partition has issues, then you probably won't be able to join the two partitions. You can try running First Aid while booted into Recovery Mode to see if the errors can be repaired, then try again to merge the partitions.

Unfortunately if the old Windows partition is showing as "Free Space", then it seems @__Bubba__ is correct and you did not use Bootcamp Assistant to remove Windows & remerge the partitions.....that is the only reliable method to do so. Otherwise, if it is showing as "Free Space", then you could try using one of the built-in macOS command line utilities (most likely "diskutil") to merge the Free Space into the macOS partition/Container. I've seen it done once, but I don't recall the command needed or the options to that command. The correct command was posted on this forum several years ago.

Before you do anything else, make sure to have a good backup since modifying partitions is extremely risky.