I have a full paid version of Snagit. now I updated as requested the software. however, the new software does not allow me to enter my old PAID key! does it mean with the new IOS the old version is not working anymore and I have to buy a new version if I upgrade my system???????????
Likely what happened is the update notification was one to install the trial for version 4 of Snagit and, if you would like to continue using Snagit 4 then you would need to purchase the upgrade. However you are welcome to uninstall Snagit 4 and return to Snagit 3 (available for download here) if you do not wish to stay on your purchased version.
I would like to point out that upgrading your OS to a new version such as going from Mac OS X 10.11 "El Capitan" to Mac OS X 10.13 "High Sierra" may be problematic, as the last version of Mac OS X that Snagit 3 was compatible with, and supported on, was Mac OS X 10.11 "El Capitan." Being on an unsupported operating system means that Technical Support will be unable to provide technical assistance for it so be cautious upgrade your operating system and good practice is to verify that your existing software is compatible with a completely new operating system, before upgrading to it.