i want to install macOS Catalina but it dosen't work

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Level 1

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i want to install macOS Catalina but it dosen't work

i am on macOS Monterey on a iMac 21.5 inch late 2015 and i tried to install macOS Catalina and then i found a macOS Catalina installer from the mac App store and i tried to install it but it was installed fine and then there was a problem. it stands that this operating system is to old. i was sad. and then i found another installer from macOS Catalina from the apple and i downloaded from the official apple but not from app store. and then again problem that the operating system is old. but what can i use to install it ???

Please im begging i need macOS Catalina back ;(

iMac 21.5″

Posted on Aug 26, 2025 2:09 AM

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Answer 1

Top-ranking reply

HWTech

Level 9

68,228 points

Aug 26, 2025 8:53 AM in response to baset290

If your Mac is currently running macOS 12.x Monterey, then you cannot run the downloaded Catalina installer because you cannot downgrade an existing OS.

Instead you must create a bootable macOS Catalina USB installer using the instructions in the following Apple article:

Create a bootable installer for macOS - Apple Support

And like @John Galt mentions, you will need to perform a clean install which involves booting to the installer and using Disk Utility to erase the whole physical drive (or the Fusion Drive item if your iMac has a Fusion Drive setup), Quit Disk Utility, and select the "Install macOS Catalina" option. Within Disk Utility you will need to click "View" and select "Show All Devices" before the physical drive (or possibly Fusion Drive) appears on the left pane of Disk Utility.

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Answer 2

John Galt

Level 10

154,880 points

Aug 26, 2025 7:27 AM in response to baset290

Start by reading How to reinstall macOS - Apple Support. In that document, it describes how to erase the Mac, which will be a prerequisite: Use Disk Utility to erase an Intel-based Mac - Apple Support. Follow those steps first.

It bears repeating that erasing the Mac will leave it completely empty — no operating system, none of your data, nothing.

Then, referring to the first document, boot Recovery mode with the key chord that corresponds to "you might get the macOS that came with your Mac, or the closest version still available." Emphasis on might because Apple has been making it difficult to obtain older macOS versions.

Depending on the macOS version it installs, then you may proceed to upgrade that version to Catalina. Refer to How to download and install macOS - Apple Support, under Catalina 10.15.

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