How can I fix the gaps between planks It was not properly done Thank you
Now sure what kind of floor you have, so here are several for serveral types of flooring:
On hardwood floors years ago, thin rope was put between the cracks. This probably would look better on an older-type hardwood floor.
I would also contact the company that installed the floor improperly to come and make things right.
I have maple hardwood in a 100+ year old house. Buy wood fill to match your hardwood colour. I then mix this with bonding agent (kind of glue). I tape all around the gap, then apply the wood fill/bonding agent mix. Squish it into the gap, and scrape off excess to use on next gap. When dry, I apply a coat of poly with a fine paint brush.