One trick is to wrap the matrices inside a framed environment and putting the location parameter there:

\starttext

\startformula
\startmatrix[left={\left(},right={\right)}]
  \NC A \NC B \NR \NC C \NC D \NR
\stopmatrix
=
\startframed[location=top,frame=no,offset=none]
\startmatrix[left={\left(},right={\right)}]
  \NC A \NC B \NR \NC C \NC D \NR
\stopmatrix
\stopframed
=
\startframed[location=bottom,frame=no,offset=none]
\startmatrix[left={\left(},right={\right)}]
  \NC A \NC B \NR \NC C \NC D \NR
\stopmatrix
\stopframed
\stopformula

\stoptext

> Am 06.01.2023 um 17:48 schrieb Jacob Kauffmann via ntg-context <ntg-context@ntg.nl>:
> 
> Signierter PGP-Teil
> Hi all,
> 
> Thank you for your recent assistance with calligraphic math fonts.
> 
> I am trying to use a matrix example from math-ali.mkxl, but I am running into an issue.
> When I try to use the location parameter, I receive the error
> 	A <box> was supposed to be here.
> 
> I have also tried both location=bottom and location=top, but both give the same error.
> 
> Do you have any ideas on how I can get the matrix in the desired locations?
> Thank you in advance for your help.
> 
> Best regards,
> Jacob
> 
> \starttext
> 
> \startformula
> \startmatrix[left={\left(},right={\right)}]
>   \NC A \NC B \NR \NC C \NC D \NR
> \stopmatrix
> =
> \startmatrix[left={\left(},right={\right)},location=low]
>   \NC A \NC B \NR \NC C \NC D \NR
> \stopmatrix
> =
> \startmatrix[left={\left(},right={\right)},location=high]
>   \NC A \NC B \NR \NC C \NC D \NR
> \stopmatrix
> \stopformula
> 
> \stoptext
> 
>