For some children 18 months is too early to potty train because some children at 18 months old are not ready or capable of grasping the toilet concept.
If the 18 month old is removing their diapers or hates being wet or poopy and can tell when they are wetting their diaper or pooping their diaper then they may be ready.
My mom says she potty trained me at 18 months and I did good.
However my own kids would not potty train at 18 months and they wore diapers until near their 4th birthday and then they suddenly decided that they would no longer want to wear diapers and wanted to go to the toilet instead of having a wet or soiled diaper.
You can certainly try to potty train the 18 month old and some will usually get the concept of the toilet at that time and if not then just wait until they are 2 years old or 3 years old and try again.
Don't force the potty training issue unless the child is 4 years old or above.
Just go back to diapers and try again later.