Unable to delete disk partition in dual boot system.

Before you do anything else, make sure you have a good backup of your macOS system just in case something goes wrong....modifying partitions is always a risky venture.

You will need to delete the partition & merge it back into the macOS partition.

Partition a physical disk in Disk Utility on Mac - Apple Support

FYI, I never recommend anyone to partition any drive for any reason because people almost always will realize one or more partitions are too small. Modifying partitions is always risky. Sometimes Disk Utility may not be able to merge the other partition back into the main macOS partition.

If you want to install Linux, then the best option is to install Linux into a Virtual Machine assuming your system has the resources to do this & the Linux workload will perform adequately in a VM. If you do need to install Linux on bare metal for better performance, then installing Linux onto an external USB3 SSD is the best option so it will minimize any risk to the main macOS system on the internal SSD.

So that we can help you best, please tell us which Mac, model and year you’re having trouble with, and what macOS version you’re running. Find that info by clicking the Apple menu () at the top-left of your screen and then select About This Mac. (Note- for privacy and security please never post your serial number.)

You may be able to use Disk Utility to delete the Ubuntu partition and recover the drive space.

Hint: before you do this, please make sure you have a current Time Machine back up of your startup drive. The above process has a high potential to wipe everything from your startup drive.

Use Time Machine to back up - Apple Support

Hi,

You can't change the size of the partition without erasing both part first. If you want to access memory though: You can reinitialize that ubuntu partition, BUT only if you erase it before (so you'll lose everything from that partition) and then convert it into a readable format from your mac ( like osx extended ).

Otherwise, you might be able access info on that volume using the terminal, but that's not ideal and practical.

Other possibility, looking into emulation program like vbox, that would allow communication with Ubuntu partition.

Formatting in Apple File System (APFS) : for OS 10.13 or later.

or Mac OS extended : for macOS 10.12 or before.