I'm 45 years old and my Vitamin D level came back at 21 - will this Vitamin D3 60,000 IU granules sachet help correct it?

A level of 21 does suggest insufficiency, and a high-dose sachet like this one is commonly used to correct it, usually taken weekly or biweekly for a set number of weeks rather than daily. The exact schedule really depends on how low your level is and whether you have any absorption issues. Your doctor can work out the right course and when to recheck your levels.