If you are on Windows try to stop/start the ArcGIS Data Store Windows Service to see if that clears things, then if the data store is still read-only try again to change it to read-write.
I am adding a link to the document as reference.
ArcGIS Data Store utility reference—ArcGIS Enterprise | Documentation for ArcGIS Enterprise
The changedatastoremode utility is also used to place the relational data store back in a read-write mode after you finish maintenance or after you add enough disk space to the primary data store machine to allow the data store to function properly in read-write mode.
You must run the changedatastoremode utility on the primary relational data store machine, not the standby machine.
Note:
If ArcGIS Data Store places your relational data store in read-only mode due to insufficient disk space, automatic backups are also disabled to avoid filling the disk further. Therefore, you must reset your automatic backup schedule using the updatebackupschedule utility after you place the relational data store back in read-write mode.
| Marcelo Marques | Principal Product Engineer | Esri |
| Cloud & Database Administrator | OCP - Oracle Certified Professional |
I work with Enterprise Geodatabases since 1997.
“ I do not fear computers. I fear the lack of them." Isaac Isimov