Can you help me find this error?
Tks,
The error is written right there 
Loop means that your network has two or more ways to reach the destination. Normally you would use something like spanning-tree-protocol (or the faster version rstp) so that your switches or bridges know where to send the packages. That requires your network equipment to support this feature.
The error in your screenshot states that your mikrotik router received a package with its own mac-address (which shouldnt be the case) thus concluding you have an active loop in your network.
You should check your setup for configuration or physical connection mistakes.
In many cases, don’t means that you have a real loop because i have some nodes with only one path and sometimes i see in log the same message as you.
Regards