Trying to update software on iMac

A 2015 iMac cannot be updated beyond macOS Monterey. See the following from Identify your iMac model - Apple Support

iMac (Retina 5K, 27-inch, Late 2015)

Model Identifier: iMac17,1

Part Numbers: MK462xx/A, MK472xx/A, MK482xx/A

Newest compatible operating system: macOS Monterey

iMac (Retina 4K, 21.5-inch, Late 2015)

Model Identifier: iMac16,2

Part Numbers: MK452xx/A

Newest compatible operating system: macOS Monterey

iMac (21.5-inch, Late 2015)

Model Identifier: iMac16,1

Part Numbers: MK142xx/A, MK442xx/A

Newest compatible operating system: macOS Monterey

iMac (Retina 5K, 27-inch, Mid 2015)

Model Identifier: iMac15,1

Part Numbers: MF885xx/A

Newest compatible operating system: macOS Monterey

Also see the following from macOS Sonoma is compatible with these computers - Apple Support

iMac

Identify your iMac

• iMac (24-inch, M1, 2021)
• iMac (Retina 5K, 27-inch, 2020)
• iMac (Retina 5K, 27-inch, 2019)
• iMac (Retina 4K, 21.5-inch, 2019)

To learn how to upgrade macOS please read Update macOS on Mac - Apple Support. A "Late 2015" model iMac can be upgraded to run macOS "Monterey".