Date: 202005
Local time of day: 1201-1800
Location: ZZZ.ARTCC, US
Aircraft: Any Unknown or Unlisted Aircraft Manufacturer
Flight plan: IFR
Flight phase: Descent
Airspace: Class E ZZZ
ATC / advisory: Center ZZZ
Second aircraft: Any Unknown or Unlisted Aircraft Manufacturer
Reporter organization: Government
Reporter function: Departure; Approach; Enroute
Reporter qualification: Air Traffic Control Fully Certified
Human factors: Troubleshooting; Workload; Situational Awareness; Time Pressure; Confusion
Second reporter function: Supervisor / CIC
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation - Altitude Excursion From Assigned Altitude; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural Published Material / Policy
Detector: Person Air Traffic Control
When detected: In-flight
Result: General None Reported / Taken
Contributing factors: Airspace Structure; Environment - Non Weather Related; Staffing; Procedure; Human Factors
Primary problem: Staffing

Narrative: I was working 3 low altitude sectors combined plus ZZZ Approach Control. It is highly unusual for us to ever be working approach airspace outside of the normal midshift times; but we are during morning rush hours to reduce ZZZ Approach Control employee's exposure due to COVID-19. We are also working ZZZ1 Approach into the morning hours. The complexity of multiple approaches and unfamiliarity with operations at ZZZ airport inside ZZZ Approach airspace led to a missed readback error while we were formulating a plan how to deal with the situations.     I had a Aircraft X northwest bound at XX0 and an Aircraft Y southwest bound at FLXX0 who needed lower to also do multiple approaches at ZZZ with all the other aircraft doing the same. I descended the Aircraft Y to XY0 above the Aircraft X and missed that he read back ZZ0. The Aircraft Y read the clearance backward which was awkward and caused me to miss the altitude readback. Instead of the Aircraft Y saying Aircraft Y descending to ZZ0 altimeter 30XX; he said 0XX ZZ0 Aircraft Y. When I heard 0XX I looked for a data block with a 0XX in it not realizing it was the Aircraft Y clipping the altimeter and saying it first so I missed the ZZ0 part.Center is working sectors combined with fewer people than normal which is fine; but now just this week they now have us doing ZZZ1 Approach and ZZZ Approach during hours we normally do not work Approach Control. The frequency congestion and lack of frequency coverage at the lower Approach Control altitudes adds an undue burden and complexity to Center normal operations which are fairly busy at times with the combined sectors and reduced staffing. We are also working those 2 approach controls earlier in the evening than normal. The approach controls need to work their normal hours and this incident would not have occurred.


Second narrative: Aircraft Y; NW bound; level at XX0.  Aircraft X ; SW bound at FLYY0 was given descent clearance to XY0.  Pilot read back ZZ0.  When the radar showed Aircraft Y at XX7 the Controller issued a climb clearance to Aircraft Y and a descent clearance to Aircraft X.  Numerous calls for coordination was being accomplished by the X-side around the time of the incident.Due to COVID-19; 3 TRACONs are on reduced operating hours; ZZZ2 TRACON; ZZZ1 TRACON; and ZZZ TRACON.  ZZZ ARTCC new specialty controllers are tasked with assuming the airspace for a longer time with a substantial increase in traffic in the morning hours.  By the X o'clock hour ZZZ2 TRACON and ZZZ1 TRACON have assumed their airspace. The issue lies with the late opening of ZZZ TRACON. There is a major increase in traffic the hour before they open with arrivals; departures; overflights; military flights; and practice approaches. These are operations that increase the complexity and mental fatigue of the controllers that are unfamiliar with conflicting approaches; missed approach procedures and coordination with towers. I recommend that ZZZ Approach still stay open for the total hours they feel will increase the safety of their workforce; however as soon as practicable shift the operating hours 1 hour earlier to control traffic their facility is accustomed to working on a daily basis. This will decrease ZZZ's Center's workload to a greater level of safety and increase the level of safety in ZZZ TRACON's airspace during this morning traffic period.


Synopsis: Center Controller and Front Line Manager reported an airborne conflict event while working Center airspace along with three other Approach Control airspaces.