Correct, the batteries are on the side of the combination lock! Here is the link to the operating instructions.
https://cdn.masterlock.com/masterlock_eu/resources/documents/pdf/LFW082FTC_LFW123FTC_LTW123GTC_LFW205FYC_LTW205GYC.pdf
Try pushing the lever upwards first, as if you were closing the safe. Perhaps the safe was not fully locked. Then enter the code again and open it. Maybe that will help :)
Hello
https://cdn.masterlock.com/masterlock_eu/resources/documents/pdf/LFW082FTC_LFW123FTC_LTW123GTC_LFW205FYC_LTW205GYC.pdf
Page 10 - Looks like the safe can only be fixed to the floor (and then it's not as waterproof).
LG
Unfortunately not. But the code is created individually and sent separately with the safe. But yes, the possibility of deactivation would be desirable.