One yellow lower leaf on a rubber tree does not establish a nutrient deficiency. Look at the whole plant, the pattern of leaf loss and what changed recently.
Is only an old leaf affected, or are several leaves changing together? Has the plant moved, become colder or stayed wet for longer than usual? Those questions narrow the investigation before a bottle of fertiliser is opened.
Inspect the pot, including the cover
Remove the inner container from any decorative cover and check for trapped water. Feel the compost below the surface. The plant's upper leaves may still look glossy while its roots are dealing with poor conditions.
Inspect for pests and note whether new growth looks sound. A photograph taken in consistent light helps distinguish an isolated old leaf from a problem moving through the plant.
Do not make several major changes at once unless the situation demands it. Correct a clear drainage or position problem, then monitor the response. The damaged leaf may still fall. The useful measure is whether the problem continues and whether later growth develops normally.
Related reading: Rubber Tree Care: Potting, Watering, Light and Feeding.