When playing solo in PoE2: Deadfire, will taking sidekicks such as Fassina or Rekke make it so I can’t get the Solo Achievement?

Hello.

To the best of my knowledge, PIllars of Eternity: Deadfire doesn't have an achievement for completing the game solo, like the various solo achievements from the original Pillars of Eternity, but is instead a toggle set when beginning a new game by clicking the head at the bottom of the menu (instead of "New Game") - a "Magran's Fire".
- I can't see such an achievement within the game itself, or as one tracked by GOG. If this is not the case for you, please, show me where this achievement is written.

The Solo option reads as follows: "Cannot recruit companions, sidekicks, or adventurers." To answer your question, the answer would be no, you can still take them if you wish, because they cannot enter your party.
But they may be somewhat of a hindrance. If recruited, they are still present aboard your ship and may participate in boarding-party fights. But they are all level one, because they cannot level up. It's actually a pain in the backside, because your companions are level one too, and can die in combat here.

Ah. Thank you for that response; I haven't played Deadfire in a while and couldn't remember if there was an achievemenfor solo play or not.