If you select "Flat," you type will result in a flat world. If you type a seed but leave the generator on "Standard," you will get mountains and rivers regardless of the seed text.
If world generation is consistently broken or crashing, use the Steam Verify Integrity tool to ensure no world-gen files are corrupted. colony survival flat world seed fix
Seeds often change between major updates (e.g., from 0.8.0 to 0.9.0). A "flat seed" from an old forum post may generate a mountain range in the current version. If you select "Flat," you type will result in a flat world