transfer from my existing licence from PC to Mac

I have a licence installed on two PC's

By @nicolas_3539

Creative Suite such as CS5, CS6?

Or Creative Cloud subscription.?

Which product(s) do you have on PCs?  Be specific.

If this is legacy Creative Suite, a platform swap is no longer possible for unsupported software.  A PC installer & license won't help you with a Mac. You either have a Mac version and Mac license or you don't. 

MODERN OPTIONS:
==============
FREE Acrobat Reader -- view, comment, print & sign PDF.
https://acrobat.adobe.com/acrobat/pdf-reader.html

Paid Acrobat Pro -- the complete solution for creating and saving PDF.
https://acrobat.adobe.com/acrobat/acrobat-pro.html

Photoshop Elements or Premiere Elements (no subscription needed, 3 year license).
https://www.adobe.com/products/photoshop-elements.html

Creative Cloud Photography Plan (approx $10/month for 12 months). Includes the following:

• Photoshop CC and Photoshop on iPad
• Camera Raw
• Lightroom on desktop, mobile and the web
• Lightroom Classic
• Portfolio website + hosting
• Adobe Express (free starter plan)
• Bridge
• 20 GB cloud storage (upgradable to 1 TB).
• Free upgrades for paid subscribers

https://www.adobe.com/creativecloud/photography.html

Premium Adobe Express ($10/month). Mobile & web-based graphics & video apps for social media content creators.
https://helpx.adobe.com/express/using/express-overview.html

Creative Cloud Bundles & Single App Plans for mobile, web & desktops
https://www.adobe.com/creativecloud/plans.html

Make sure your computer meets the minimum CPU & GPU requirements. Most Creative Cloud apps work on:

• Windows 11 (64-bit) versions 23H2, 22H2; Windows 10 versions 22H2, 21H2;
• MacOS 14.7 (Sonoma), 13.6.7 (Ventura), 12.7.6 (Monterey).

- https://helpx.adobe.com/creative-cloud/system-requirements.html

if the license is for perpetually licensed apps (eg, pse or pre-cc apps), you can't transfer the license unless it's explicitly a dual os license.

if the license is for cc, no problem.  you can install the cc desktop app from here https://creative.adobe.com/products/creative-cloud on as many (mac and/or win) computers as you own and use it to install all (or some, if you want) of your cc subscription programs.

the only (insignificant for internet connected computers) limitation is that you can sign-in (and therefore start your adobe programs) on, at most, two computers without signing-out. because you can sign out from either (or both) signed-in computers using a 3rd computer, this could not (imo) be easier.

https://helpx.adobe.com/download-install/using/install-apps-number-of-computers.html#:~:text=Activat....

I have a licence installed on two PC's but I would like to switch one of the licence from one of the old PC to my new Mac. 

I have desactivate the licence on one of the PC but I cannot put it on my Mac because the licence is for Windows. 

Is it possible that I cannot do it? 

If No, what is the procedure? `Thanks

By @nicolas_3539

If you are subscribed to the creative cloud you should be able to install the software on 2 computers regardless of whether they are Mac or PC.

If you are trying to change an older non-subscription license from a Windows license to a Mac license, that is no longer possible. There is no longer support for the older software with regards to de-/activations and there are no more license transfers to switch your PC licensed version to a Mac licensed version. But as mentioned, the new subscription would offer you some flexibility in that respect to install on the platform of your choosing.