How to change X axis in combo chart to show all 12 months instead of axis repeating months

Difficult to debug or make specific suggestions from a picture. My guess is that your horizontal axis is using 30 (days) as the major unit. So, assuming the first division is 1 Jan, the second division will be 31 Jan, the third division will be 1 Mar, the fourth division will be 31 Mar, and so on (each division exactly 30 days after the previous division). Of course, your numbers/dates are formatted to only show month and year, so you don't see the day of the month for each division.

It appears that you would like to have the horizontal axis be a nice "date" axis. Details will depend on exactly how you have created the existing combination chart. Worst case, it will involve adding a "dummy" series that you can set to a line chart type, then use the line serie's horizontal axis as a date axis. Something like in this tutorial: https://peltiertech.com/line-xy-combination-charts/

Does that help?

This example uses an additional series as a column chart.
The secondary axis can be set to Date.

You will need to set both primary and secondary axis so they share the same Minimum and Maximum value.
Depending on you excel version you should be able to copy the date and paste and excel will automatically convert to number