Unable to download latest macOS version on my iMac

Have you installed and run any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your Mac?

Give this a try: boot into Safe Mode according to How to use safe mode on your Mac and try downloading and installing from there.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

Which iMac? Is it one that is compatible with Sonoma – or with the latest version of macOS, Sequoia?

macOS Sonoma is compatible with these computers - Apple Support

macOS Sequoia is compatible with these computers - Apple Support

The current macOS is Sequoia, not Sonoma.

So far, it seems that you have not told us exactly what mac model you have.

Go to the Apple menu and choose About this Mac

Tell us what it says, including the year. Not just "iMac", but including the year, eg. "iMac (24 inch, 2023)" or whatnot.

Please read the following user tip and restate your question/problem in detail: 

     Writing an Effective Apple Support Communities Question - Apple Community

What software?

From where?

What happens when you try?