This is really frustrating. Maybe they realized their free product was too good and had to make it suck.
“Beginning March 16, 2021, LastPass Free will include access on one device type of your choice. The first device you login with on or after March 16 will set your active device type.”
So if your first device to use LastPass is a phone, then from then on only mobile devices can use your account. And vice versa if your first device is a desktop/laptop.