What does this message mean in STA report :

Warning: signoff.sdc line 104, 'top_I.branch[28].block[15].um_I.<http://block_28_15.tt|block_28_15.tt>_um_I/buf_out_uo_out_0/X' is not a valid start point.

? Because at line 104 you find :

group_path    -from $all_pins_um_ow -to $all_pads_out    -name io_outward
set_max_delay -from $all_pins_um_ow -to $all_pads_out    12.5

And looking at the report I find things like :

Startpoint: top_I.branch[28].block[15].um_I.block_28_15.tt_um_I/buf_out_uo_out_0/X
            (internal pin)
Endpoint: gpio_out[24] (output port)
Path Group: io_outward
Path Type: max

Fanout         Cap        Slew       Delay        Time   Description
---------------------------------------------------------------------------------------------
     2    0.002728    0.028618    0.000000    0.000000 ^ top_I.branch[28].block[15].um_I.block_28_15.tt_um_I/buf_out_uo_out_0/X (sky130_fd_sc_hd__clkbuf_2)
                                                         top_I.branch[28].block[15].um_I.ow[0] (net)
                      0.028618    0.000000    0.000000 ^ top_I.branch[28].mux_I/block[12].genblk1.mux4_I[0].cell0_I/A3 
...

And the above is exactly the path I wanted to constrain, and it's been constrained just fine, starting at the starting point I wanted ... So I'm not exactly sure what's that warning about ?