This can happen if you do not have the enough capacity in your work center. For example : There is a field named "Capacity" in your work center form. If you have filled the capacity as 1, then only one operation can happen at a time at that particular work center. So It's possible that you already have some unfinished manufacturing order at that particular work center. You can increase the "capacity" by editing the work center form or you can find the manufacturing order which is using that work center and complete that MO. Then you can plan a new MO at that work center.
Hope this helps..!!
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Thy setting in problematic mrp.production: date_planned_start several years ahead; (2023, 2024, 2025)
I had the same problem, and when changed to planned date to 2025, it found a slot in 2027.
This is because "plan_hours" method, would iterate in search for a slot, 100 times in intervals of 14 days (so 1400 days, 3,8 years)
If it doesn't find a spot in that interval it would raise that error.