Can't update Mac Mini past 10.13.6

I just installed an internal SSD for my late 2014 Mac mini and got OS installed up to 10.13.6 but there is no option to update it any further. Software update shows nothing and I know it can go higher (I have Catalina on an identical Mac Mini that I installed the same SSD on, I just let it do it's thing when it told me there were software updates and it got to Catalina but this other Mac Mini seems to think 10.13.6 is the latest.

How can I make it update?

 SSD is from Macsales if that matters Aura Pro X2 480GB Both Mini's are the same model and same drive.