Getting A message stating macOS Monterey is not currently supported, please advise

Where are you getting such a message? It seems dubious.

Did you try upgrading to Monterey, or did the message appear out of that context?

A late 2015 mac CAN run Monterey.

But always have a full backup before doing a major OS upgrade like this.

I've had better luck from the Safe Mode because I evidently have some 3rd party software that interferes with the installation so try the following:

#1 - boot into Safe Mode according to Start up your Mac in safe mode - Apple Support.

NOTE: 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

#2 - go to the System/Software Update preference pane and begin the download.

#3 - run the installer from the Safe Mode.

Point # 1A - There are reports appearing on the ASC Forums regarding failed installation of Monterey on older Qualifying Apple Computers. In most cases this is related to having a NON Apple Original internal drive ?  The EFI Boot ROM included in Monterey is verifying for Original Apple Drive and if not found , may Balk or fail to upgrade to Monterey. Reports indicate the EFI BOOT ROM is Required to Flash the Computer and is a  requirement for a Successful Monterey Installation.

Do you have any third party antivirus or "cleaner" installed in this mac?

Please run Etrecheck and post its full report here. Use the "additional text" button and paste the report into the text box.

Etrecheck is a diagnostic utility made by one of the most respected members of this community.

You can download it free from the respective site.

That's a bit weird.

How much free space do you have on your system drive?

Are you trying to install it on a 3rd party SSD?

1991SERVR wrote:

How would I check if I am

Are you attempting to install it to an external drive and/or did you at any point replace the original hard drive?