@tomleslie 
My help also says: "If T is a triangle, and A a vertex of T, FindAngle (A, T) returns the internal angle of T at A".

@vv 
Yes, thanks, checked. I used to think that VectorAngle calculates the angle in a similar way, that is, no more than Pi / 2.
 

Fixed comments in the text of the program, 

@Kitonum 
Yuri Nikolaevich,  you should to agree that the question is not quite that.
And you saw an example of my solution on one of the forums so that you can compare with your example.

It's about the simplicity of the general approach, as is the case with inellipse in this topic.

There is a similar program (as at the beginning of the post), but for obtaining the equation of an ellipse circumscribed about a triangle.
Is there a way to parametrically define the circumellipse too, so as not to fiddle with this program further?

@vv 
And it works for me too, but when the parameters are  0 < s, t < 1
For example, parameters = [s = 0.01 .. 0.99, t = 0.01 .. 0.99]

@mmcdara 
Thank you for your assistance. The error message ceased to appear, the first frame and sliders appeared. But when I move the sliders, the triangle and ellipse images disappear and no longer appear. That is, it still does not work.
Nothing, the main thing is not in this, but in the fact that the essence of solving the task by means of parameterization is clear.

@vv 
Thank. Until now, I did not know about the existence of such dependencies.
" Probably in Maple 17 this will work:"
No, for some reason, it doesn't work. But this is no longer relevant, because it is clear what should be.
Thank you again.
(You will laugh when you find out how much effort I put into this program, especially with my Maple "proficiency" level.)

@vv  
I feel that you, as always, are on top. But unfortunately your program doesn't work on my version.
Is it not difficult for you to at least briefly explain your text? Thank you.

@vv  
Yes, there are several ways to solve this problem. The fact is that in some similar examples there is only a finite number of variants of such triangles, and it seemed to me that the easiest way to decide (for fun) is to use the optimization package. 
For example, x^4+ y^4 + z^4 - 1 = 0, the maximum area of the triangles is approximately 2.08.

It seems  there is a way to exit the program when a check for the existence condition of the tetrahedron is "NO". We put the program text in a pseudo-loop where the break statement works. If the check passes, then the  print ("GO") is executed; if not, the print ("GO") is not executed. A simple check by point F: if its third coordinate is other than 0, then the program is executed, for example, point (F, 1, 1, 0.000001), if its third coordinate is 0, then the program is not executed.

restart: with(geom3d):
for j to 1 do 
point(A, 0, 0, 0), point(B, 2, 2, 0), point(C, 0, 2, 0), point(F, 1, 1, 0):   
gtetrahedron(T1, [A, B, C, F]):
i := nops(op(0, detail(T1))); if i = 1 then print("Ok, gtetrahedron") else    print("No gtetrahedron, NO"); break end if:
print("GO");
end do:

@tomleslie  Thanks a lot. I didn’t want to extend the text of the check, especially since I couldn’t do it as gracefully as you. I have very poor Maple proficiency. I was more interested in understanding how to stop further program execution in the case of `no_tetrahedron_possible`

@SaeedAF7 
I think the author of the topic simply wanted that to him be shown  how to establish uniform rotation of one of the vertical struts of the mechanism (input link of mechanism).
I don't have MapleSim either.

@Rouben Rostamian  

 You wrote: “I don't understand your code for making a cylinder. ”
plots [arrow] when head_length = 0 will look like a cylinder. In this case, the cylinder diameter is width=1, and the length of the cylinder is equal to the distance between the middles of the segments with red points. The direction of the arrow is also set along the axis of the cylinder.

What about Markiyan Hirnyk  and MaplePrimes Search?