Photoshop CS 5.1 not working on my new iMac (10/28/21)

My Photoshop was a bought 10 years ago as a disc in 2010 or 2011. It is CS5.1, I moved over all my data and PS did not work, I read that it is not going t work, I hate buy a monthly subscription that I will not use for 2-3 months, then use for 3-4 hours, then not use again. I use it to cut out the background when taking photos of my products and listing them on Amazon.com.

I wish I could copy it to an external hard drive and use it that way. 

Any help is very much appreciated. 

Kirk

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