Bootcamp

hi, i am new to mac and i recently used bootcamp and booted into windows, made a partition in windows, and when i wanted to erase windows from mac it says that 'the startup disk cannot be partitioned or restored to a single partition'. i have attached the photo. please please help me

The error message you received says it all. You partitioned the drive while using Windows, so now you can not restore it back to one partition using the Boot Camp Assistant. You will have to use Disk Utility to remove the partiton and restore the drive to a single partition. Disk Utility is destructive so make sure you backup your Mac data as well as your Windows data.

My answer to you is generic because you did not tell us anything about your Mac (model, year, etc) nor which version of macOS it's running.