I have a 1099-B that doesn't specify 'short term' or 'long term' covered stocks? What is the difference and what should i choose as an option? Thank you

Short term: An asset is held for one year or less, then sold.

Long term: An asset is held for more than one year, then sold. 

This distinction of "covered" and "noncovered" occurred as a result of a law pass in January 2011 where the brokerage was then required to report cost basis for shares to you and the IRS. for shares acquired after January 2011.  The term "covered" is used for this reporting. For shares acquired prior to January 2011 there is no brokerage reporting requirement. Hence the term "noncovered".

From Vanguard:

For tax-reporting purposes, the difference between covered and noncovered shares is this: For covered shares, we're required to report cost basis to both you and the IRS. For noncovered shares, the cost basis reporting is sent only to you. You are responsible for reporting the sale of noncovered shares.

covered shares are shares your broker tells you to report with Box A or Box D checked on Form 8949.

BUT

As an active investor, be aware that your category Box A or Box D sales without adjustments do not require Form 8949, so there is no reason to import or key in those transactions.
Instead use the "enter a summary" option to put your numbers on Schedule D Line 1a or Line 8a.