You must enter a verifiable tax ID number (Employer Identification Number or Social Security Number) in order to use Stripe.
Even when using an Employer Identification Number (EIN) to verify a business, you are still required to provide the Social Security Number (SSN) for the individuals associated with every US Stripe account.
If you do not have an EIN and are an individual (not an LLC, Sole Proprietor, etc.) you can generally use your SSN in place of the EIN.
If you are unsure what number to enter, consult a tax professional.
If you have recently applied for a tax ID number in the US but Stripe was unable to match the TIN against government records, see: New Tax ID Number for US business not verifying.