“I have been living in South Africa with my son and daughter since 2003 and have been advised not to travel to Zimbabwe due to my health condition, require special diet and medication which is not readily available in Zimbabwe.”
especially “…, require…” or "… which/that requires "
You should try to make your sentences shorter. You could try to subdivide this run-on line into 2 or 3 individual sentences, and then we’ll take it from there
“… advised not to travel to Zimbabwe due to my health condition, which requires a special diet and medication that is not readily available in Zimbabwe.”
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Although the word ‘diet’ can be used uncountably, that sort of usage would have to be an extremely general sort of usage. I think it would be safe to say that the word ‘diet’ is usually used countably: