Fractional ffmpeg timecode in draw text
The timecode_rate value is rounded and then applied, so 0.2857 gets rounded down to 0, which is considered invalid. That error message should be refined.

Since you seem to want timecode value to increase by 0.2857 after each 32 frames, you can supply a custom text value

text='%{e\:0.2857*n/32}'


Remove the timecode and timecode_rate options.