No sandbox user '_apt' on the system, can not drop privileges

Add the _apt user back to /etc/passwd and /etc/shadow:

$ sudo grep _apt /etc/passwd /etc/shadow
/etc/passwd:_apt:x:168:65534::/nonexistent:/bin/false
/etc/shadow:_apt:*:17121:0:99999:7:::

YMMV: The _apt UID on my system is 168, and yours may not be 168 on your system - the value should be unique, and greater than 100.

You can re-create the _apt user by using the following command:

sudo adduser _apt --force-badname

It doesn't matter what you put for the password or any other fields.

Then, apt will continue to work as normal.

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