mothmanh i know this is may be a dumb question but how big is your EFI boot partition ? do you have any other operating systems besides Manjaro and Solus installed i.e. Windows 10 etc. if not , make sure that efi boot partition is at least 512 mb. . if it is smaller , say 300 mb that is most likely your problem.
if you have manjaro up and running install efibootmgr program in manjaro . it is a command line terminal program . i have only used it once so i don't recall the exact command to use in the program to increase or free up space on the EFI boot partition . you should be able to find out using Google search engine.
sorry i cannot be of further help. maybe someone else can
Good luck and welcome to Solus .
