Yes. An SLR/DSLR does not, by definition, require interchangeable lenses—“single-lens reflex” only means the viewfinder sees through the taking lens via a reflex mirror or similar splitter. So a fixed-lens SLR is technically possible, and examples have existed. One digital example mentioned is the Olympus E-10. On the film side, the Canon EX series is a partial example: it had a built-in rear lens section with interchangeable front elements.

They’re uncommon because a reflex camera’s main advantages—through-the-lens optical viewing and support for different focal lengths—make the added size, complexity, and cost of the mirror box and prism easier to justify when lenses can be changed. If the lens is fixed, those penalties often make less sense, especially once electronic viewfinders and mirrorless designs became viable. Fixed-lens large-sensor cameras are therefore more often mirrorless, because removing the reflex mechanism allows a smaller, simpler camera while keeping high image quality.