To load FM23 and FM24 save games in FM26, first, you'll need to manually move the save games to FM26 by following these steps:
1. In Desktop Mode, click the folder in the bottom left corner
2. Under 'Devices' select 'Home'
3. In the top right corner, click on the dropdown menu and make sure 'Show Hidden Files' is ticked
4. Copy the save games you wish to load in FM26:
FM23: Deck/Local/Share/Steam/steamapps/compatdata/1904540/pfx/drive_c/users/steamuser/Documents/Sports Interactive/Football Manager 2023/Games
FM24:
Deck/Local/Share/Steam/steamapps/compatdata/2252570/pfx/drive_c/users/steamuser/Documents/Sports Interactive/Football Manager 2024/Games
5. Paste the save games to Deck/Local/Share/Steam/steamapps/compatdata/3551340/pfx/drive_c/users/steamuser/Documents/Sports Interactive/Football Manager 26/Games
6. Launch FM26 and load the save game as you normally would (the process could take a few minutes)
If nothing above works, please submit a support request stating exactly what you've tried so far, and a member of our team will try to help.