"STR: Command not found" while assigning a value to a variable

I entered the following lines into a shell script called “test.sh”:

#!/bin/bash
echo Hello World
STR=”Hello Again”
echo $STR

After chmod to executable, I ran the script with ./test.sh. However, I get the message

./test.sh: line 3  STR: Command not found

What am I doing wrong?

What are you using to create that script? The quote characters you have in your question are wrong. They're not regular double quotes (") but ”. The regular quotes are:

$ uniprops '"' | head -1
U+0022 ‹"› \N{QUOTATION MARK}

While yours are:

$ uniprops '”' | head -1
U+201D ‹”› \N{RIGHT DOUBLE QUOTATION MARK}

So, just use regular quotes and you'll be fine:

#!/bin/bash
echo Hello World
STR="Hello Again"
echo "$STR" ## Get into the habit of ALWAYS quoting your variables as well