Remove unallocated free space from Mac

Hello,

I decided to remove the Windows partition from my Mac, but I noticed that the space occupied by this partition was not merged with the main Macintosh partition, but appears as an unallocated free space. It is as if the removal of the Windows partition had also removed the space that was occupied by this partition, so on the Mac I still have a small hard disk, as if that Windows partition were still present.

Basically, at the beginning I had a hard disk of about 121 GB, after the partition the Mac was 83 GB and the Windows partition 37 GB. After removing the partition, I have a Mac of 83 GB instead of 121 GB.

I also send you some screenshots to make you better understand. You can see the free space of 37 GB and the Mac of 83 GB. The procedure would be to select the free space and decide to remove it in order to have a single partition of 121 GB as it was before. I did it, but it gives me error.

Do you know how to solve the issue?

Thank you so much!