At $1.75 million, the Pats rank second to last in the NFL in available cap space.
This article does a nice job summarizing. Tom Brady leaving (he chose to leave, he put up his house on the market in Aug'18), and Brady's original cap hits all accelerated to hit the Pats with $14M in dead money at once. Cutting AB (thanks Kraft) and several others also created dead money. They only had $1M to sign Cam at the time remember. Then later that summer a crazy high 8 players in the covid-19 opt-out left including Hightower, Chung, Cannon. So anyways this was always going to be a rebuilding year to clean the cap and set up for a strong 2021.
Just putting this all in perspective. Some need to chill out the same way it was unfair to pile on when Tom Brady completely didn't realize it was 4th down.