Just to add some information to @aibd’s good answer: Indeed, Google changed some time ago the way the Google Play Services signature was checked. Hence, a corresponding patch had to be implemented for microG (the open source Google Play Services replacement used by /e/OS and other custom ROMs). The patch was rolled out with /e/OS 2.6.x. (And indeed, I have just installed Google Maps on my OnePlus 6 with /e/OS 2.6.3, and Google Maps seems to work as intended.)
Unfortunately, you seem to be currently stuck at /e/OS 1.2 or /e/OS 1.20. The OTA upgrade is currently not available due to the Murena.io outage (and it is not clear whether it will be available at all even after the outage).
There are several threads with the fp4 label and some information how a “dirty flash” upgrade can be done without data and app loss, e.g.
If you decide not to wait for the end of the Murena.io outage and to try a dirty flash when back home, you should still backup all your data beforehand because the upgrade could still fail. (There is the hard saying that all data without backup is by definition unimportant and can be deleted without regret.)