1. Lewis base definition
   A Lewis base is a species that can donate a lone pair of electrons.

So, we check whether each compound has an available lone pair on the central atom that can be donated.

2. Examine each option

A. $SF_4$

Sulfur has 6 valence electrons. In $SF_4$:

• 4 electrons are used in forming 4 $S-F$ bonds
• 1 lone pair remains on sulfur

Thus, $SF_4$ has a lone pair available and can act as a Lewis base.

B. $NF_3$

Nitrogen has 5 valence electrons. In $NF_3$:

• 3 electrons are used in forming 3 $N-F$ bonds
• 1 lone pair remains on nitrogen

Thus, $NF_3$ has a lone pair available and can act as a Lewis base.

C. $ClF_3$

Chlorine has 7 valence electrons. In $ClF_3$:

• 3 electrons are used in forming 3 $Cl-F$ bonds
• 2 lone pairs remain on chlorine

Thus, $ClF_3$ has lone pairs available and can act as a Lewis base.

D. $PCl_5$

Phosphorus has 5 valence electrons. In $PCl_5$:

• All 5 valence electrons are involved in forming 5 $P-Cl$ bonds
• No lone pair remains on phosphorus

Since there is no lone pair available for donation, $PCl_5$ cannot act as a Lewis base.

3. Conclusion The compound that cannot act as a Lewis base is:

$\boxed{PCl_5}$

So, the correct option is D.