Deuteronomy 4:2

Ye shall not add to the words that I teach you nor diminish them, but keep the commandments of the Lord your God which I command you.

You shall not add i.e., you shall not add your own ideas, thinking that you thereby serve God. Likewise, you shall not diminish.

YE SHALL NOT ADD [UNTO THE WORD WHICH I COMMAND YOU] — “such as placing five sections [of the Torah] in the phylacteries, five species in a lulav, are: the palm-branch, the ethrog, the myrtle and the willows. — For phylacteries See Vol. II, p. 168, Note 406. five fringes [in a garment]. The same principle applies to the prohibition, neither shall ye diminish from it” [by placing three sections of the Torah in the phylacteries etc.]. This is Rashi’s language. And thus the Rabbis said in the Sifre: 82. “Whence do we know that you are not to add to [the four species in the commandment of] the lulav, are: the palm-branch, the ethrog, the myrtle and the willows. — For phylacteries See Vol. II, p. 168, Note 406. nor to the [four] fringes? Scripture therefore says, Ye shall not add. And whence do we know that you are not to diminish from them? Because Scripture states, neither shall ye diminish. Whence do we know that if [a priest] began to recite the Priestly Blessing he is not to say: ‘Since I began to bless [the people], I will [further] say, The Eternal, the G-d of your fathers, make you a thousand times so many more as ye are?’ Because Scripture states, [Ye shall not add] unto the ‘word’ — even ‘a word’ you are not to add to it.” However, not only concerning these [commandments] have the Rabbis said [that the prohibitions neither to add nor to diminish, apply], but also in the case of someone who sleeps in a Booth on the eighth Day [of the Festival of Tabernacles]. Thus the Torah specifically states that the commandment to dwell in Booths applies to seven days only, and not to the eighth Day, which is a separate festival (Shemini Atzereth). Sleeping is part of the commandment “to dwell” in a Booth. [However, because of the general rule that he who feels uncomfortable is exempt from sitting in a Booth, sleeping there, in the colder climates in which we live, is practiced only by those who are very strict in religious observance (Orach Chayim 639:2, R’ma).] with the intent [of performing a religious duty] that he is liable to whipping [for having added to the commandment], as is mentioned in Tractate Rosh Hashanah. So also if he were to observe the Festival of Tabernacles for only six days, he would be transgressing this negative commandment [neither shall ye diminish from it].
In my opinion, even if someone devised an independent commandment [rather than …

ולא תגרעו ממנו לשמור, a person should not make the mistake made by King Solomon that he can ignore a prohibition since he knows the reason for the prohibition and can create conditions when such a prohibition would not be justified. [the Torah had forbidden a king to marry too many women, warning that these could lead him astray. Solomon relied on his wisdom not to lead him astray and ignored the Torah’s law with disastrous consequences for his kingdom. Ed.]

לא תוסיפו וגו, “do not add, etc.;” G–d implies that anyone adding to the words of the Torah,-however well meaning he may be,-will in fact detract from the value of the Torah. [If G–d is perfect, something that every believing Jew accepts as axiomatic, any addition to or deletion from His Torah would make it, ergo Him, imperfect. Ed.] For instance, when the Torah decreed 4 strings of tzitzit for the corners of a four-cornered garment, adding a fifth string would not only not make it holier, but would make it useless, and anyone reciting a benediction over such a tallit would desecrate the holy name of G–d by pronouncing such a benediction. The same reasoning applies to sitting in a sukkah for an extra day or adding another a fifth species to a lulav.