There is no direct connection from Cattolica to Serravalle. However, you can take the line 44 bus to S. Maria del Piano 1 then take the drive to Serravalle. Alternatively, you can drive from Cattolica to Serravalle in around 27 min.
How to get from Cattolica to San Marino
From Allende 1 line 44 bus to S. Maria del Piano 1 then drive to Serravalle. 45 min 4€. From Cattolica drive to Serravalle. 27 min 4€.