Rules of adding "-erlessness" to words? [closed]

"-less" and "-ness" are both suffixes. A Suffix can be attached to the end of a word to modify it.

"-less" at the end of a word means "without".

Example:

Fearless-- "My dog is not afraid of anything. He is fearless."

Thankless - "She works hard but never receives praise. Her job is thankless."

"-ness" at the end of a word refers to a quality or state of being.

Example:

Weakness - The baby goat could not stand on his own yet, so his mother compensated for his weakness.

Darkness - He was unable to see anything without any light. He was surrounded by darkness.

Suffixes can also be combined. To use one of your examples, thoughtlessness would refer to the state of being without thought.