Episode 10 of Countdown ends Volchek's arc and leaps 10 months into the future, showing how the case and its aftermath have changed the force's lives. With Nathan coming home to his children and wife, Meachum finally gets a clean bill of health regarding his medical condition and Fitz, the newcomer, investigates some low-priority cases from the "Muzzle Pile". Fitz links a case with a high-profile national emergency, and that is when the entire task force once again comes together to solve another mystery, except for Amber Oliveras.
Amber Oliveras doesn’t return to the task force in Episode 10 of Countdown because she’s reassigned to a DEA case. She’s officially working her own assignment and can't be called back for the new mission. This appears as a deliberate narrative choice reflecting her professional duty and the emotional distance forming between her and Meachum, especially when they shared a heartwarming moment.
Here is why DEA Agent Oliveras did not return for the task force's new mission in Countdown Episode 10

Episode 10 showcases DEA agent Oliveras and Meachum spending a night together before Meachum begins his first medical trial the day after. Meachum never wanted Oliveras to stop living her life because of his conditions; however, the next day, when he is heading for his first treatment, Oliveras gathers the entire task force to support him, and Meachum walks in with trust and courage.
The show then takes a 10-month leap into the future, and as mentioned, Nathan's special task force heads on to another national terror case. Fitz, while reviewing the Muzzle cases, encounters an email with a voice note from a woman turns out to be Patricia Perez. She mentions something about the remote cabin, and when Fitz investigates further, he finds out about the murder of Perez. The team then finds out some guns, a manifesto, maps and even a burnt photograph of the US president. This makes them understand the gravity of the case and come together to embark on yet another journey.
Meachum, after getting a clean chit from the hospital, joins the team, but Amber Oliveras doesn't seem to be part of the team. This is because she is busy with another assignment. This narrative choice enables the show to portray the friction between Meachum and Amber's relationship, leading to an emotional distance between the characters.
What could it mean for Amber and the task force in Countdown?

The Creator of Countdown, Derek Haas, confirms, while speaking with Decider, that, while Amber isn’t back on the team at the end of Episode 10, fans can still expect more Meachum–Oliveras screen time ahead.
“I will say that at the end of 10, Oliveras has not rejoined the team, but you are going to see more Meachum-Oliveras scenes. Because those two actors, they’re magic and chemistry was so awesome that it was really fun to watch it come to life, and so you’re gonna see more of that.”
By having Amber pulled off the team due to her DEA assignment, Countdown underscores that these characters are part of a larger law-enforcement ecosystem, sometimes that means being pulled away from the main storyline, even when personal feelings are involved. It adds realism and raises the emotional stakes.