[rospack] Error: stack/package roscpp not found

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What is causing this issue?

With:

export ROS_PACKAGE_PATH=~/<PATH TO THE DIRECTORY I WAS MAKING THE PACKAGE IN>

you are overwriting the ROS_PACKAGE_PATH variable, so that it only points to ~/<PATH TO THE DIRECTORY I WAS MAKING THE PACKAGE IN>. The default paths needed for ROS to function are now gone, hence the error message you received.

If you really must change/update ROS_PACKAGE_PATH manually, be sure to only add to it. Something like:

export ROS_PACKAGE_PATH=/home/user/path/to/dir/i/was/making/pkg/in:$ROS_PACKAGE_PATH

This inserts the new path and copies in the old entries.

How do i solve it?

Depending on how you setup your system, simply opening a new terminal could restore the old ROS_PACKAGE_PATH values. Alternatively, source /opt/ros/hydro/setup.bash should also set the variable to its default value.