Prior to Windows 11, Microsoft designed Windows versions to run on as many PC's as possible.  This only makes sense as they wanted everyone to adopt the new system.  Windows 11 is a change in this tradition, and it will not run on many PC's that are active in use and getting the job done.  This does not mean that this equipment is out of date and/or needs replacement; it simply means that this perfectly good system, that is otherwise getting the job done, will not run Windows 11.

As a general rule, you need a system with an 8th Gen CPU or better to install and update Windows 11.  There are some tricks in the trade that will allow you to install Windows 11 on lesser equipment but it will not work as Microsoft intended and these work-arounds are not recommended.