What is the address where your coils starts with?
Type in Codesys help Modbus addresses - there is a table with addressing.
For example in the slave %MX0.260.0 corresponds to start address 2080 in the master COM_MOD_MAST
Also COM_FCT:=1 for bits reading, COM_FCT:=15 for bits writing
In FCT 3, you have to read n words.
If you want to read booleans, Convert the booleans to word ,
For example , In abb PLC Pack library is available it converts 8 bits to 1 byte. like that you will create a PACK program convert bool to word , then transmit the word through communiction .
In receiving end , In abb PLC UNPack library is available it converts 1 BYTE to 8 bit.
like that library you will create 1 program & then unpack your word and use it as a bit.