Reports can only be pinned to your dashboard if they are run with dynamic dates. Dynamic dates update automatically as time progresses.
For example, MTD or Month-to-date will always be the current month and will automatically roll over into a new month, but 7/1-7/31 range report will always be July and not update when the month is over. If you're missing the icon, make sure your report is set to a dynamic date and not fixed dates.