I cannot start my Flash CS5 yersion in subscription, only trial

https://www.adobe.com/products/flashplayer/end-of-life.html was 12-31-2020

It seems there was a product called "Flash CS5.5 Subscription Edition".  This was years before Creative Cloud subscriptions. Here Adobe mention it: https://helpx.adobe.com/uk/flash/release-note/flash-professional-cs5-5-release.html

Here in 2011 an article talks about it and gives prices: https://www.htmlgoodies.com/webmaster/adobe-creative-suite-software/ - Master Collection $129 per month, single app Flash Pro $35 per month. I see prices have come down!

Anyway, a question for the original poster - are you trying to install Flash Pro Subscription, having kept paying your subscription since 2011?  The same $35 per month since then?

When looing at your question, I was confused by why it is saying 'subscription' for a Creative Suite (CS).  Subscription only applies to applies to Creative Cloud (CC) products. Do you have any CC prodicts installed?

Looking further at this, I cound a thread that looks to address your issue - How do I prevent Creative Cloud from taking over my CS6 perpetual license?
https://community.adobe.com/t5/download-install/how-do-i-prevent-creative-cloud-from-taking-over-my-cs6-perpetual-license/td-p/6491847