Can I install Photoshop if my Mac doesn't support Big Sur or above?

Unfortunately, Adobe says that Big Sur (11.x) or later is required and "Installation is blocked on v10.15.x" That's inconsistent as far as I'm concerned. I installed Photoshop CC on Catalina a year ago and all is working fine, including the latest updates! However, that pertains to the recently released new (v24).

The previous version, Photoshop 2022 (version 23.x), is compatible with Catalina. You could attempt installing the trial version of the Photography Plan to see if it will sense that you have Catalina (15.7) and install Photoshop v23 instead of v24. If not, there does appear to be a way to install older versions of the CC apps through the Adobe Creative Cloud app; you will need to create an account (free) to get there.

All you can do is contact the vendor to see if they provide an older version of Photoshop the will run on your computer.