It may be because you are friends with Murat already, currently the game for some reason only lets you befriend either Murat, Marmont, Davout or Ney.
The only exceptions are Lannes and Dumas, which have different friendship flags.
Edit: Checked the files and here is how to befriend him in detail, you start with either +5 friendship score if you travelled to Egypt with him and Napoleon on L'Orient or +4 if not, you need 10 or more to become his friend.
Here are all the ways you can raise it:
| When/where | Choice | Score |
| Deciding the fate of El-Koraïm | “Marmont. You decide.” | +1 |
| Stranded sailors | Equip them as a Naval Infantry battalion | +1 |
| Troop discipline | Pay the troops from your own funds | +1 |
| Bedouin | Bribe some Bedouin to turn coat | +1 |
| Slavery policy | “Marmont is right. Tax the slave trade out of existence.” | +1 |
| Any city improvement | Pay the 5000 | +1 |
| Menou expedition | Send Marmont with a strong escort | +1 |
| Menou expedition | Strip the garrison bare and send Marmont | +2 |
| Ottoman blockade | Either just deny that the Sultan declared war, or pass the discretion check | +1 |
| Bombardment aftermath | Either fund and oversee repairs or just help with the repairs without paying | +1 |
| If Alexandria revolts | Shoot the ringleaders | +1 |
And here are the choices where it is lowered:
| When/where | Choice | Score |
| Stranded sailors | Send sailors to Cairo | -1 |
| Discipline | Publicly shoot worst offenders | -1 |
| Menou expedition | Send nobody or the Naval brigade | -1 |
| Savants ti Cairo order | Send nobody | -1 |
| Ottoman blockade | Tell the truth | -1 |
| Bombardment aftermath | Leave rebuilding to others | -1 |
| If rebellion happens | Torture or parole prisoners | -1 |
| Delegate everything to Marmont | What it says on the tin, delegating all your Decisions to him | -2 |