EKF2_AID_MASK should be set to 1 for GPS.
Have you set CP_DIST > 0? From the warning, it looks like you have the avoidance system on and it may be seeing something.
How are you mounting the GPS unit? Just an un-modified RB5 with mast? I assume you're outdoors with a clear view of the sky. In some cases, interference will throw that error. Setting COM_ARM_WO_GPS will always allow you to arm despite GPS errors. If you still can't arm, there's usualy another issue, maybe as stated above that the obstacle avoidance system is enabled. Another place to check is in QGC under safeties to ensure the avoidance system is disabled.
Againg for the GPS< Flashing blue, means it's still waiting for the GPS to be fully ready. Small blue LED on the side will flash blue showing a lock.
You can also try opening up the GPS check values or disabling some checks. For instance you can open up ekf2_req_sacc from 0.5 to about 0.75.
There's also the GPS check you can experiment with to see what exactly the error is to get to ready (most likely interference). You can try turning off bit 4 for instance. Your mask is currently set to the default 245 or 11110101.