Languages play many roles in a society. Because of this they have evolved to have some pretty fine distinctions about things. “Drunk” vs “intoxicated”, “retard” vs “mentally disabled”. These are registers and exist to paint a picture. Maybe you want to be intentionally rude (to offend the other person, or because you don’t care if you offend them or not). Japanese and other languages have ways of being polite that are baked right into the grammar.
I wrote that paragraph to give you an idea about some of the smaller details in a language that can make it complex. All languages are complex.
There is no “easy” language. The whole idea of an easy language is a complete misnomer. English is not easy, neither is Spanish, Swedish, Arabic, Chinese or Navajo.
Some can be EASIER than others based on your native language but not language is easy. Esperanto is not easy for a native speaker of an non European language (and neither is Atlaans for that matter). Keep in mind that your native language has a huge effect on what you consider easy.
Languages evolved to provide humans with a way of communicating. So they have evolved to make use of our cognitive abilities. There is no natural language out there that is too complex for a human to speak because it was made by humans in a natural process.
Some languages might have harder pronunciation, harder and more inconsistent spelling, but the complexity overall evens out. If you find a language has easy grammar, it will have complexity in another area.
Literally, the only people who can’t learn a language are people that are actually mentally disabled (in the medical sense, not using it as an insult), but they couldn’t learn any language because they don’t have normal cognitive abilities.