Short answer: sometimes. Available research and practitioner reports indicate the combination is not always prohibited, but the psilocybin experience is often muted or different while on an SSRI. Effects vary by person and by medication.
Common approaches people consider include continuing the SSRI and accepting a likely dampened effect, tapering then waiting about five half-lives as a rule of thumb, or tapering slowly, stabilizing, then planning a session once things feel settled. The best fit depends on the specific SSRI, dose, duration, and sensitivity, with fluoxetine often requiring the most time due to its long-acting metabolite.
Abrupt changes can bring sleep or anxiety issues that may color a session, so many prefer gradual pacing. Discuss any changes with your prescriber. Substances that rely on MAO inhibition or strong serotonin release, like ayahuasca or MDMA, are commonly avoided with SSRIs.
For a practical overview of these options and SSRI specific timing notes, see this summary. I cannot verify your individual situation, so treat this as general information only.
Hope this helps.