Please check Automaticly find a good Plaintext Modulus At a high level, the procedure for finding a plaintext modulus that is compatible with batching is explained on Slide 15 of the FHE integer arithmetic webinar at Homomorphic Encryption for OpenFHE Users – OpenFHE.org
But this is needed only if you want to support component-wise multiplications (what batched encoding typically refers to). In the case of additions only, a different encoding can be used that does not have the constraint you mentioned. For example, p=2 would be allowed. In this case, you would use MakeCoefPackedPlaintext instead of MakePackedPlaintext when encoding the input vectors. See Template Class CryptoContextImpl — OpenFHE documentation for more details (or search for code examples/unit tests in the library).