you can skip the SCSI2SD firmware update, it's not mandatory, it can work even if it has not the latest version
1 download the OS 7.5.5 from here
https://www.savagetaylor.com/wp-content/uploads/68k_Macintosh/Bootdisks/755_2GB_volume.zip
2 unzip the archive
3 use balenaetcher to transfer the image file to the SD, it must be > 2GB
4 insert the SD in the SCSI2SD
5 connect the SCSI2SD to your modern host PC/Mac.
6 Configure SCSI2SD using the tool here
http://www.codesrc.com/files/scsi2sd/, chose the right card you own. Take care of configuring the right SCSI ID, usually 0 if the HD is not connected or not present, ID 1 works fine as well (you can do this just once if you do not need to change the ID). You can skip this step if it troubles you, but please disconnect the HD from the Color Classic Mac to avoid conflicts with the ID
7 connect the SCSI2SD card to your not
powered-on Mac either internally or extenally
8 turn it on
other variables can impede the successful boot, but in case you must be vey specific and detailed in describing where it failed and what are the symptoms, "having troubles with" or "I got stuck" does not allow us to help you further, in case you can send a video or pictures
all the best