Are graves that are built as domes above ground with a height of one and a half meters, in which the deceased is buried after 60 cm of sand is placed and excavated, legitimate and permissible for burial, or is it obligatory to bury in the legitimate lahd (niche), and what if the legitimate lahd is not available?

The fundamental principle is to bury the deceased within the ground, and it is impermissible to bury them above it unless absolutely necessary. If you are able to secure a grave for burial in the prescribed manner, then that is obligatory. Otherwise, there is no harm in purchasing this grave. Graves must not be raised above the ground by more than one handspan.