Hi , @automating
Can you try to use this code ? In flow , there is no & in the expression.
concat( substring(outputs('Get_response_details')?['body/FirstnameID'], 0, 1) , '.', outputs('Get_response_details')?['body/LastnameID'] )
If this can not help you , can you show the error code in your side?
If this reply can help you , you can click mark this reply as solution (Accept solution) which can help more people, thanks in advance!
Best Regards,
Yueyun Zhang