#!/bin/bash
s='14_611646T,C
14_881226CT,C
14_861416.1GGC,GGCGCGCGCG'

sed 's/\(.*[0-9]\)\([^0-9]\)/\1 \2/' <<< "$s"
sed -E 's/(.*[0-9])([^0-9])/\1 \2/' <<< "$s"