Have you got this error in AutoCAD running some command or app that is AutoLISP based?
First thing to ensure is that Visual LISP is loaded so run (vl-load-com).
Below shows a result that is expected and all is working. Note that the drawing needs to have at least one object in it.
#<VLA-OBJECT IAcadLine 0000005ce02af728>
If you get below result then something is broken in AutoCAD.
; error: no function definition: VLAX-ENAME->VLA-OBJECT
I have not got this problem myself but a repair or reinstallation of AutoCAD has helped a couple of customers, one with AutoCAD 2016 and another with AutoCAD 2017. Suspected cause of this could be having run some registry cleaner or anti-virus that deleted vital registry settings that AutoCAD needs. One had run Malwarebytes the day earlier.
There might be an easier remedy but until this is found you know what to do to solve this problem. If you haven’t install any recent service pack or hotfix (that comes with an installer) this might also be a solution.