Just added SSD to old Macbook pro - its asking me to select disk to install Mac OS X? its not allowing me to progress

You need to launch Disk Utility and erase the physical drive with GUID partition and MacOS Extended (Journaled). If the physical drive is not shown in the left pane of Disk Utility, then click on "View" and select "Show All Devices".

@Allan Jones, I'm not sure what happened to your post as all that is now shown is a dot.

Allan Jones wrote:

What OS version? I believe it is recommended to use APFS for an SSD if you are running High Sierra 10.13:

File system formats available in Disk Utility on

Mac - Apple Support

When I installed an SSD in my 2012 MBP I formatted it APFS, cloned the old drive to the new one, and it booted first try.

If the system doesn't have the new firmware update yet, then HFS+ is needed for the installer so it can apply the necessary firmware update for the system. The installer will automatically convert the drive to APFS. The only way to have High Sierra or newer use HFS+ for the boot SSD is by modifying the USB installer to force HFS+. I just figured using HFS+ was the safe option since the end results will be the same.

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I was interrupted while looking for some links I did not have bookmarked and, when I got back, I saw the OP was in good hands--yours! I nulled my response.