I am not expert with AM and I currently use AM+, but this is how I have setup up an "emulator" to run Sonic Mania.
Create 'C:\Emulation\attract-mode\emulators\Sonic Mania.cfg' and add below text, then save. (use your own folders and filenames to match).
# Generated by Attract-Mode Plus v3.0.6
#
executable C:\Games\Sonic Mania\SonicMania.exe
workdir C:\Games\Sonic Mania\
system Sonic Mania
info_source thegamesdb.net
artwork box C:\Games\Sonic Mania\snap
artwork boxart C:\Games\Sonic Mania\snap
artwork cart C:\Games\Sonic Mania\flyer
artwork cartart C:\Games\Sonic Mania\flyer
artwork flyer C:\Games\Sonic Mania\flyer
artwork marquee C:\Games\Sonic Mania\marquee
artwork snap C:\Games\Sonic Mania\video;C:\Games\Sonic Mania\snap
artwork wheel C:\Games\Sonic Mania\wheel
Use AM to then setup an emulator with the name Sonic Mania.