Bought LR 6 stand alone. Did everything I needed for business. Until I HAD to get subscription.

And yes - the subscription you would want is called the Photographer's or Photography bundle, which includes Lightroom and Photoshop and more!!!

And unless you erased your main drive at some point, you probably have the presets you purchased and your watermarks in a folder somewhere on that drive. 

I have another suggestion:

Instead of installing the old Mojave system on your mac, why not get an external drive. Install Mojave on that drive, make it a boot drive, and then install the disk etc. for LR 6 onto that boot drive.

You can then do your Lightroom work when booted into the external drive, and also use the newer operating system when you need it for everything else, by simply rebooting into the main drive.

I can not acress my watermarks, and can only create NEW watermarks in Photoshop 

How did you create the old watermarks?

• Bring up Adobe.com
• Sign in (you may be already signed in)
• Upper right, your icon (profile picture) for your account, click on it, click on "View Account"
• Scroll down to "Your Plans and Products". What Plan?

Ok, my bad, read reply's twice, answer once, you no longer have the subscription. Ok, but to understand, while at Adobe.com, what does purchase history state for the plan you had?

Since I'm a Windows user, I will refrain from making a recommendation other than you would really do well to subscribe to the Adobe photography plan and install Lightroom Classic, let it upgrade your existing catalog and go to work. It is a much more advanced Lightroom than Lightroom 6, and I think you would be quite happy with it. If you want to stay with Lightroom 6 then keep watching. I'm sure someone will be able to recommend which operating system you will have to go back to in order to install it.

Lightroom 6 will NOT install on a Mac computer running the Big Sur operating system. Although Lightroom 6 is a 64-bit application, the installer is 32-bit and Big Sur will not run the installer. There is no alternative to installing Lightroom 6. You can't have Lightroom 6 on that computer using that operating system because Lightroom 6 is too old & too out of date. You are going to have to choose  some other alternative, whether that be Adobe or some other vendor.

What Mac operating system are you running? IF you are able to run the installer that is on the disk it will only install Lightroom 6.0. There were 14 updates that were made available. But first, please verify what OS you are using.

It's possible that you chose the wrong subscription plan that includes only  Lightroom (the cloud version) and doesn't include Photoshop. From the description of your needs, you probably should have subscribed to the photography plan which includes Lightroom Classic, Lightroom and Photoshop. Lightroom Classic is the step up from Lightroom 6 and is probably the version that you would find most beneficial for what you plan to use it for. Lightroom is the cloud version, and is somewhat limited feature wise when compared to Lighhtroom Classic. In short, I think you chose the wrong plan.

No one forces you to get a subscription. LR 6 still runs on most Mac operating systems. (Can't be installed on the newer operating systems, but it runs).

This subscription version is very limited without photochop 

Don't know what you mean. Lightroom Classic subscription includes Photoshop. Maybe you bought the wrong subscription, you want the Photography Plan, not the Lightroom Plan.