We know the even perfect number formula is $2^{p-1}(2^p − 1)$ and it is known that the multiplication of a even number and odd number is a even number. So why can't we say there are infinitely many perfect numbers since $2^p-1$ has to be a prime number in $2^{p-1}(2^p − 1)$ and it is known that there are infinitely many prime numbers?
Note: I'm not a native english speaker.
On
Because $2^{p}-1$ is not always a prime. When it is a prime, it is called a Mersenne prime.
For example, $11$ is prime, but $2^{11}-1 = 2047 = 23 \times 89$.
If $2^p - 1$ is a prime (often called a Mersenne prime), then $2^{p-1}(2^p - 1)$ is a perfect number. While there are infinitely many primes, it is not known whether there are infinitely many Mersenne primes. There are many primes which cannot be written in the form $2^p - 1$, for example, the prime $5$.