ddcutil requires read/write access to /dev/i2c devices, or more precisely all /dev/i2c devices associated with video cards.
On some distributions, package i2c-tools creates group i2c, and assigns that group to the /dev/i2c devices using a udev rule. In that case all that is necessary is to add your user name to group i2c:
$ sudo usermod your-user-name -aG i2c
If group i2c does not already exist, the following command will create it:
$ sudo groupadd --system i2c
Then add your userid to group i2c as above.
A sample udev rule for giving group i2c RW permission on the /dev/i2c devices can be found in distributed file 45-ddcutil-i2c.rules. Its exact location varies by distribution, but commonly the file is found in directory /usr/share/ddcutil/data. The file can be copied to /etc/udev/rules.d, but do check that this rule does not conflict with others in that directory.
$ sudo cp /usr/share/ddcutil/data/45-ddcutil-i2c.rules /etc/udev/rules.d
For testing, it may be simpler to give everyone permission to write to /dev/i2c-* for the current boot:
$ sudo chmod a+rw /dev/i2c-*
For USB devices, see USB Device Permissions