I want to download photoshop 2020 ,because I have old desktop PC ,but I don't see anywhere

no, it's not possible with the cc app.  but if you contact adobe, you can get a link to the 2020 apps.

using a browser that allows popups and cookies, contact adobe support by clicking here, https://helpx.adobe.com/contact.html?rghtup=autoOpen
in the chat field (lower right), type AGENT
be patient, it can take quite a while to reach a human

p.s. if you're contacted by anyone (via email or private message), it's much more likely to be a scammer than an adobe representative. ie, double check for an employee badge if contacted in the forums and look for an adobe.com domain in the email address if you click reply to an email. then check again and be very suspicious. any other method of contacting (or offering to contact you) is almost certainly a scam, https://community.adobe.com/t5/using-the-community-discussions/staying-safe-in-the-adobe-support-community/td-p/12919684/redirect_from_archived_page/true.

p.p.s you can also use phone (but chat is usually quicker), https://helpx.adobe.com/contact/phone.html

p.p.p.s. you can also use twitter to tweet @AdobeCare.