Question: Is the Name Allah one of the 99 Beautiful Names of Allahu ta’âlâ? What does it mean?
ANSWER
Yes, this sacred Name is the first of Allahu ta’âlâ’s Names.

It is Allahu ta’âlâ’s personal and special name, which encompasses the meaning of each Name. In this respect, it comprehends all beautiful qualities and divine attributes contained in His other Names. His other Names, however, refer only to the meanings they carry. For this reason, not any other Name can substitute for the name Allah. This Name cannot be applied, even in the figurative sense, to anyone else other than Allah. But it is permissible to give humans some of the other Names of His.

The 99 Names of Allahu ta’âlâ are called Asmâ-ul Husnâ. The Names of Allahu ta’âlâ are dependent upon Islam’s prescription. He is called by the Names which Islam has dictated and referred to. Islam has not permitted us to call Him or to refer to Him with any Names other than these.
(The explanation of Birgivî Vasiyyetnâmesi)

It is a sin to call Allahu ta’âlâ God. It is a grave and ugly crime not to be willing to use the Name Allah and to use the name God or even one of the 99 Names instead of Allah. (Endless Bliss)

In other words, it is not permissible to be intentionally unwilling to use the Name Allah and to use another Name instead. For example, one cannot use deliberately even such Names as Rabb, Rahman, and Haqq in order to avoid using the Name Allah. That is, herein what is considered to be an offence is not to want to use the Name Allah. It is an offence not to want to use the Name Allah and to use His other Beautiful Names instead. Or else, it is not an inconvenience to use His other Beautiful Names in addition to the Name Allah.

Whenever uttering, hearing, or writing the name of Allahu ta’âlâ, it is wâjib to say or to write when it first occurs, and it is mustahab if it is used more than once to say or to write a word of reverence, such as Subhânallah, Tabârakallah, Jalla jalâluh, Azza ismuh, Jallat qudratuh, or Ta’âlâ.