AutoCAD 2016 is not easily available at Autodesk anymore: do you not have a copy of that expensive software you purchased stored somewhere safe to use?
AutoCAD2016 is not supported in Windows11. That means Autodesk is never going to offer a fix/solution.
AutoCAD 2016 SP1 update is required for it to work in WIn10 (and by implication, Win11): do you have that download too?
this site offers free (and not so free) tips on getting old AutoCAD to work in newer Windows version, explore there for more ideas
https://blog.longbowsoftware.com/ Best of luck.
