open source     > level 1, order 1, name 'cont-yes.mkiv'
system          > 
system          > ConTeXt  ver: 2020.05.25 23:39 MKIV beta  fmt: 2020.5.27  int: english/english
system          > 
system          > 'cont-new.mkiv' loaded
open source     > level 2, order 2, name 'V:/tmp/context/tex/texmf-context/tex/context/base/mkiv/cont-new.mkiv'
system          > beware: some patches loaded from cont-new.mkiv
close source    > level 2, order 2, name 'V:/tmp/context/tex/texmf-context/tex/context/base/mkiv/cont-new.mkiv'
system          > files > jobname 'rotation', input './rotation', result 'rotation'
fonts           > latin modern fonts are not preloaded
languages       > language 'en' is active
open source     > level 2, order 3, name './rotation.tex'
fonts           > preloading latin modern fonts (second stage)
fonts           > 'fallback modern-designsize rm 12pt' is loaded

tex error       > tex error on line 2 in file ./rotation.tex: ! Illegal unit of measure (pt inserted)

<to be read again> 
\relax 
\grph_rotate_finish_yes ...f_analyzerotate \numexpr \p_rotation_rotation \relax 
                                                                                \d_grph_rotate_width \d_grph_rotate_height \d_grp...
\grph_rotate_finish_indeed ...h_rotate_finish_nop \else \grph_rotate_finish_yes 
                                                                                \fi \egroup 
\syst_boxes_with_next_box ->\grph_rotate_finish_indeed 
                                                       \egroup 
l.2 \rotate[rotation=-0.2]{Rotation}
                                  

1     \starttext
2 >>  \rotate[rotation=-0.2]{Rotation}
3     \stoptext
4     


Dimensions can be in units of em, ex, in, pt, pc, cm, mm, dd, cc, nd, nc, bp, or
sp; but yours is a new one! I'll assume that you meant to say pt, for printer's
points. two letters.