You can use the printf command, like this:
printf "%-10s |\n" "$txt"
The %s means to interpret the argument as string, and the -10 tells it to left justify to width 10 (negative numbers mean left justify while positive numbers justify to the right). The \n is required to print a newline, since printf doesn’t add one implicitly.
Note that man printf briefly describes this command, but the full format documentation can be found in the C function man page in man 3 printf.