Stuck in locked bootloader

Hey all, hope everyone is having a great day (I'm not),
I own a SM-T510 Samsung Galaxy Tab A 10.1 WiFi.
It all started when I tried to flash Nexus and it worked partially, but I didn't gain access to my internal storage for some reason, so I reverted back to stock rom.
Then for some reason part of the nexus rom was still there, but I was afraid of damaging my device further.
For some reason, netflix wasn't working, so I tried to apply a software update to my device using Smart Switch.
The update didn't work and I was caught in a Nexus bootloop. I entered download mode and I tried flashing twrp using odin. I thought it worked until I realised that it wouldn't get past the teamwin screen.
I re-entered download mode and there it gave me an option to factory reset the device while locking the bootloader. This is where I screwed up as I thought what else could possibly happen to my device?
SO I locked my bootloader and whatever I did I couldn't get out of download mode. AND here I screwed up even more. I tried to flash the stock ROM again using odin as a last resort but it just wouldn't complete after 10 minutes, so I closed Odin and restarted my device and here I am, stuck in a screen that says, "An error has occurred while updating the device software. Use the Emergency recovery function in the Smart Switch PC software." Now obviously this wouldn't work because my device isn't even listed there. But to note, my device is recognised by Odin and my PC.
Note: I am an absolute noobie at rooting so be easy on me.
Note 2: adb and fastboot doesn't work for me for some reason, my powershell says, "The term 'fastboot' is not recognized as the name of a cmdlet, function, script file, or operable program.
Check the spelling of the name, or if a path was included, verify that the path is correct and try again."
Thank you all for any help. I am at my wit's end.