Welcome in the forum, Ahmed.
For every bit (byte) the SPI interface gets, one bit (byte) is returned immediately. When the very first byte is sent, the interface does not "know" yet what to answer, so a dummy byte is returned which should be skipped.
SPI has no read command, so you must send dummy bytes to retrieve the information wanted.
A pitfall is the select line, which is automatically taken low when a byte is sent. When the buffer is empty it is taken high again. This can lead to interface errors when the byte sequence is not provided fast enough resulting in ss-line glitches.
Your WriteByteArray() will not work, because sizeof(data) is always 4 which is the size of a pointer.
In Read_Reg() the number of bytes written do not match the number of bytes read.
It is always more helpful when you post your complete project, so that we all can have a look at all of your settings. To do so, use
Creator->File->Create Workspace Bundle (minimal)
and attach the resulting file.
Renamed SPIM_ReadByte(), SPIM_WriteByte()APIs to SPIM_ReadRxData(), SPIM_WriteTxData()
SPIM_ReadRxData() API returns invalid data if the FIFO is empty. Call SPIM_GetRxBufferSize(), and if it returns a nonzero value then it is safe to call the SPIM_ReadRxData() function.