DSolve::ivhead
NDSolve::ivhead
NDSolveValue::ivhead
ParametricNDSolve::ivhead
ParametricNDSolveValue::ivhead
RSolve::ivhead
Here x occurs in the head of x[0] in the second equation:
DSolve[{f'[x] == f[x], x[0] == 1}, f[x], x]This shows valid input to DSolve:
DSolve[{f'[x] == f[x], f[0] == 1}, f[x], x]