  A search for a narrow scalar resonance decaying into an opposite-sign muon
pair produced in events with and without $b$-tagged jets is presented in this
paper. The search uses 36.1 fb$^{-1}$ of $\sqrt{s}= 13$ TeV proton-proton
collision data recorded by the ATLAS experiment at the LHC. No significant
excess of events above the expected Standard Model background is observed in
the investigated mass range of 0.2 to 1.0 TeV. The observed upper limits at
95$\%$ confidence level on the cross section times branching ratio for
$b$-quark associated production and gluon-gluon fusion are between 1.9 and 41
fb and 1.6 and 44 fb respectively, which is consistent with expectations.
