My 2020 return is almost done. I'm ready to finish it, but now can't do so online. My Mac is too old to for 10.14 so I can't use the software. Help!

If you cannot use the downloaded program then your options are :

1) seek another DIY company that still have a working 2020 online program 

2) find a local tax pro

3) get the forms and instructions from the fed/state and completing them manually

4) find a friend/family member that can let you borrow their computer

Did you pay for Online?  Then you can ask for the Desktop program to continue and finish.  Maybe you can ask a family or friend with a Windows or Mac computer you can use.  Otherwise you can't use Turbo Tax.  You can get the blank forms for free from the IRS and fill them out by hand.  Or go to a local accountant or tax place.