Without seeing your config, I assume that you have something (mangle rules, routing rules) that makes LANs use specific WAN. Otherwise there would be no problem and router would be happily routing between LANs. So it's probably something with that, and you want to exclude LAN->LAN traffic. How exactly, it depends on what you have now. Another possible explanations is firewall blocking the traffic.
Thank you for your response here is my settings, am sure there is something wrong , please help me out :
[admin@MikroTik] > export compact
# jun/18/2022 02:53:12 by RouterOS 6.49.6
# software id = 32W0-9I7B
#
# model = RB750Gr3
/interface ethernet
set [ find default-name=ether1 ] name=ether1-WAN1
set [ find default-name=ether2 ] name=ether2-WAN2
set [ find default-name=ether3 ] name=ether3-LAN1
set [ find default-name=ether4 ] name=ether4-LAN2
set [ find default-name=ether5 ] name=ether5-LAN3
/interface list
add name=WAN
add name=LAN
/interface wireless security-profiles
set [ find default=yes ] supplicant-identity=MikroTik
/ip pool
add name=dhcp_pool0 ranges=192.168.1.20-192.168.1.254
add name=dhcp_pool1 ranges=192.168.2.20-192.168.2.254
/ip dhcp-server
add address-pool=dhcp_pool0 disabled=no interface=ether3-LAN1 name=dhcp1
add address-pool=dhcp_pool1 disabled=no interface=ether4-LAN2 name=dhcp2
/interface list member
add interface=ether1-WAN1 list=WAN
add list=LAN
/ip address
add address=192.168.1.1/24 interface=ether3-LAN1 network=192.168.1.0
add address=192.168.2.1/24 interface=ether4-LAN2 network=192.168.2.0
add address=209.146.21.18/23 interface=ether1-WAN1 network=209.146.20.0
/ip dhcp-client
add interface=ether1-WAN1
add disabled=no interface=ether2-WAN2
/ip dhcp-server network
add address=192.168.1.0/24 dns-server=8.8.8.8,8.8.4.4,192.168.2.2 gateway=192.168.1.1
add address=192.168.2.0/24 dns-server=8.8.8.8,8.8.4.4,192.168.2.2 gateway=192.168.2.1
/ip firewall nat
add action=masquerade chain=srcnat out-interface=ether1-WAN1
add action=masquerade chain=srcnat out-interface=ether2-WAN2
/ip route
add distance=1 gateway=209.146.21.17
/system clock
set time-zone-name=Asia/Manila
[admin@MikroTik] >