Solution to DefaultRuntimeDir must be a valid directory Error

If the default runtime folder for Apache is incorrect or does not exist, it is normal to get the error in the title.

First, check if the /var/run/apache2 folder exists.

mkdir /var/run/apache2
If the folder already exists

source /etc/apache2/envvars
We can call the commands in the envvars file and start it again.

Finally, you can check again with apache2 -V.