Because the reagent AlCl3 (the Lewis acid which is used as a catalyst in friedel crafts reaction)
being electron deficient acts as a Lewis base.
And attack on the lone pair of nitrogen present in aniline to form an insoluble complex which precipitates out and the reaction does not proceed...

$$Hope it's help... $$

Why Aniline does not undergo Friedel-Crafts reaction?

Aniline is a primary aromatic amine with a lone pair of electrons on the nitrogen atom. This lone pair of electrons on the nitrogen atom of aniline makes it a strong nucleophile. Due to this nucleophilic nature, aniline does not undergo Friedel-Crafts reaction.

Friedel-Crafts reaction

Friedel-Crafts reaction is a type of electrophilic aromatic substitution reaction where an electrophile attacks the aromatic ring. In the case of aniline, the lone pair of electrons on the nitrogen atom can coordinate with the Lewis acid catalyst (such as AlCl3) used in the Friedel-Crafts reaction, forming a stable intermediate complex. This complex deactivates the aromatic ring towards further electrophilic substitution reactions.

Formation of a stable complex

The lone pair of electrons on the nitrogen atom of aniline can coordinate with the Lewis acid catalyst, forming a stable complex that inhibits the attack of electrophiles on the aromatic ring. This stable complex prevents the formation of the desired product in the Friedel-Crafts reaction.

Substitution at the para position

If aniline were to undergo Friedel-Crafts reaction, substitution would occur at the para position of the aromatic ring due to the deactivating effect of the nitrogen atom. However, the formation of the stable complex prevents this substitution from occurring.

In conclusion, the nucleophilic nature of aniline due to the lone pair of electrons on the nitrogen atom prevents it from undergoing Friedel-Crafts reaction by forming a stable complex with the Lewis acid catalyst.