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Your syntax has many problems:

• remove spaces around "=" when setting a variable

  wrong:

    APP_ID = value
  

  right:

    APP_ID=value
  

• add spaces around everything when using "[", and you need a semicolon or newline after the "]". This is because "[" is a command also known as "test" (see man test), which just like other commands, takes its arguments separated by space:

  wrong:

    if[x!= y] then echo hello world fi
  

  right:

    if [ x != y ]; then echo hello world; fi
  

• use double quotes, not single quotes when you want to expand a variable

  wrong:

    if [ '' != '${APP_ID}' ]; then
  

  right:

    if [ '' != "${APP_ID}" ]; then
  

• Also for the above example, you can use -n (non-empty) instead of comparing to empty string

    if [ -n "${APP_ID}" ]; then
  

• And for the ps example, you don't need grep and awk:

    APP_ID=$(ps -C whoami --no-header --format 'pid')
  

And so here is the fixed up script:

APP_ID=$(ps -C whoami --no-header --format 'pid')

if [ -n "${APP_ID}" ]; then
    echo "Stopping instance $APP_ID"
fi