Something is printed inside the barcode's quiet zone. The scanner cannot find the start of the symbol.
Move the neighbouring element away, or move the barcode.
^XA^PW812^BY3^FO100,50^BCN,100,Y,N,N^FD012345678905^FS^FO85,50^GB3,120,3^FS^XZ
Any printed mark (text, ^GB frame, graphic) falls inside the barcode's quiet zone.
The bar width is outside the GS1-128 range (0.495 mm to 0.940 mm, 0.495 mm target). At 203 dpi that means ^BY 4 or greater.
Raise ^BY to at least 4 at 203 dpi, or 6 at 300 dpi.
At 203 dpi ^BY2 — the power-up default — and ^BY3 both fail GS1.
This GS1-128 is shorter than the 31.75 mm GS1 requires. It may scan on a handheld and fail at a fixed scanner.
Set the height to at least 254 dots at 203 dpi, or 375 at 300 dpi.
This GS1-128 is missing the leading FNC1. Without it the symbol is a plain Code 128 and the scanner will not read the AIs.
Add FNC1 (>8) immediately after the subset start code, or use ^BC mode D.
This ^GB is thicker than it is wide or tall, so it prints as a solid black block — and if it sits beside a barcode it destroys the quiet zone.
Reduce the border thickness, or draw the block deliberately.
This example does not currently trip BC-04 under the default 203 dpi GS1 profile. That is worth knowing rather than hiding — see the open items in the repository.
Highest real-world scan-failure yield of any rule in this catalogue. Build the renderer for this one.