I can't update from catalina 10.15.7 to High Sierra 10.13, why?

All 2012 MacBook Pros can run High Sierra (10.13) – but you can't downgrade Catalina to High Sierra in place. You would be probably be looking at doing a "clean install" onto an empty external drive. (This is assuming that there's nothing in Catalina to intentionally block you from doing a "clean install" of a downgrade.)

The main reasons to downgrade from Catalina to High Sierra would be

• to run 32-bit applications (e.g., old games that were abandoned by their vendors)
• to access old CDs that use the HFS (no "+") filesystem (e.g., CDs containing old family photos)

and for either of those purposes, you might want to dual-boot, running High Sierra only when necessary.

How to download and install macOS - Apple Support

If, by chance, you were trying to upgrade to Ventura (13), no 2012 MacBook Pro can run that. For all 2012 MacBook Pros, Catalina is the end of the line.

Dvifuentes2001 wrote:

Hello fellows there. I am trying to update my catalina 10.15.7 to High Sierra 10.13.

In my experience, downgrades are not easy. Every one I have seen required wiping the drive and reinstalling macOS from scratch.

If you really want to downgrade from Catalina to High Sierra, you should make a bootable flash drive and install High Sierra from it. (Download High Sierra & create a bootable High Sierra installer.) Follow these articles for the procedure:

• How to download and install macOS - Apple Support
• Create a bootable installer - Apple Support

Alternatively, you might consider making a dual-boot system by installing a fresh copy of High Sierra on an external drive. That way you could preserve your existing Catalina system and also have a High Sierra system.

What is the reason you want to downgrade?

Catalina is newer than High Sierra.

(251240)

What if you did try to upgrade? I needed some Rt patches so I downloaded Open core Legacy processor in seen that there was an update and my 20 1513 inch MacBook Pro updated to Sonoma. It was bad now I’m trying to downgrade with a clean install I have a USB drive and have no idea what to do next.

Ajawg wrote:

What if you did try to upgrade? I needed some Rt patches so I downloaded Open core Legacy processor in seen that there was an update and my 20 1513 inch MacBook Pro updated to Sonoma. It was bad now I’m trying to downgrade with a clean install I have a USB drive and have no idea what to do next.

So you used a third-party tool to modify macOS, to install an unsupported version of macOS on your machine? That didn't work out, and now you're having trouble downgrading?

That sounds like a different issue than the one the OP is having.