"It is not heavy."
重い ( = omoi) & 重たい ( = omotai) both have the same meaning: “heavy” . We don’t usually distinguish between them very much but I think 重たい ( = omotai）is a bit stronger than 重い ( = omoi).
重い ( = omoi) is objective and 重たい ( = omotai) is subjective. So if you carry it yourself and feel the weight, you use 重たい ( = omotai)
Japanese -i adjectives have to change form to reflect negation and tense, so you must change 重い (omoi) to 重くない (omokunai).