This is a sneaky one. When creating or editing a hatch pattern, under "Island Detection," there is a setting called "Maximum Island Count" (sometimes labeled in older versions as "Hatch Limits"). This value expects an integer between 1 and 20,000. If you accidentally click into this field and press space or enter a letter, you’ll trigger the prompt.
When defining a block attribute, you can set a default value. If that default is an integer field and you type a number outside 1–20000, the error appears. autocad please enter an integer from 1 to 20000
If you or a colleague wrote a custom script that prompts for an integer range, the developer might have explicitly set the range to 1–20000 to avoid bad data. This is a sneaky one