The FYTA Terra automatically detects whether NiMH rechargeable batteries or standard single-use batteries are inserted, and disables the charging circuit for single-use batteries to prevent damage. However, this detection can be unreliable if NiMH batteries are inserted with a very low charge level.
If you see the message "Solar charging is currently off. It works with rechargeable batteries only.", fully charge your NiMH batteries externally first (e.g. using a separate battery charger), then insert them into the FYTA Terra. The message should disappear and solar charging should be active again.
Note: If you are using non-rechargeable batteries (e.g. standard alkaline batteries), solar charging is intentionally disabled - this is by design.