How to create a partition on an external hard disc and mount it in linux when all the partitions and the partition table are erased?
In the usual way. Just connect the external disk drive to the Linux machine and verify it shows up in the output of lsblk. Then you can format it via fdisk (or any other partitioning tool), and then create a filesystem on it via mkfs. Eventually, you mount it and the disk is ready to use.